diff --git a/lib/start-proxy.js b/lib/start-proxy.js
index 902413ef5..0477ff1a5 100644
--- a/lib/start-proxy.js
+++ b/lib/start-proxy.js
@@ -87,6 +87,9 @@ function getCredentials(logger, registrySecrets, registriesCredentials, language
     }
     const out = [];
     for (const e of parsed) {
+        if (e === null || typeof e !== "object") {
+            throw new util_1.ConfigurationError("Invalid credentials - must be an object");
+        }
         // Mask credentials to reduce chance of accidental leakage in logs.
         if (e.password !== undefined) {
             core.setSecret(e.password);
diff --git a/lib/start-proxy.js.map b/lib/start-proxy.js.map
index ed5747d0a..458718a8d 100644
--- a/lib/start-proxy.js.map
+++ b/lib/start-proxy.js.map
@@ -1 +1 @@
-{"version":3,"file":"start-proxy.js","sourceRoot":"","sources":["../src/start-proxy.ts"],"names":[],"mappings":";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;AAgCA,wCA0FC;AA1HD,oDAAsC;AAEtC,2CAAsD;AAEtD,iCAA4C;AAW5C,MAAM,yBAAyB,GAA6B;IAC1D,IAAI,EAAE,kBAAkB;IACxB,MAAM,EAAE,YAAY;IACpB,UAAU,EAAE,cAAc;IAC1B,MAAM,EAAE,cAAc;IACtB,IAAI,EAAE,iBAAiB;IACvB,IAAI,EAAE,gBAAgB;IACtB,EAAE,EAAE,gBAAgB;IACpB,oFAAoF;IACpF,OAAO,EAAE,EAAE;IACX,GAAG,EAAE,EAAE;IACP,KAAK,EAAE,EAAE;CACD,CAAC;AAEX,kEAAkE;AAClE,+DAA+D;AAC/D,gDAAgD;AAChD,SAAgB,cAAc,CAC5B,MAAc,EACd,eAAmC,EACnC,qBAAyC,EACzC,cAAkC;IAElC,MAAM,QAAQ,GAAG,cAAc,CAAC,CAAC,CAAC,IAAA,yBAAa,EAAC,cAAc,CAAC,CAAC,CAAC,CAAC,SAAS,CAAC;IAC5E,MAAM,uBAAuB,GAAG,QAAQ;QACtC,CAAC,CAAC,yBAAyB,CAAC,QAAQ,CAAC;QACrC,CAAC,CAAC,SAAS,CAAC;IAEd,IAAI,cAAsB,CAAC;IAC3B,IAAI,qBAAqB,KAAK,SAAS,EAAE,CAAC;QACxC,MAAM,CAAC,IAAI,CAAC,qCAAqC,CAAC,CAAC;QACnD,cAAc,GAAG,MAAM,CAAC,IAAI,CAAC,qBAAqB,EAAE,QAAQ,CAAC,CAAC,QAAQ,EAAE,CAAC;IAC3E,CAAC;SAAM,IAAI,eAAe,KAAK,SAAS,EAAE,CAAC;QACzC,MAAM,CAAC,IAAI,CAAC,+BAA+B,CAAC,CAAC;QAC7C,cAAc,GAAG,eAAe,CAAC;IACnC,CAAC;SAAM,CAAC;QACN,MAAM,CAAC,IAAI,CAAC,yBAAyB,CAAC,CAAC;QACvC,OAAO,EAAE,CAAC;IACZ,CAAC;IAED,qCAAqC;IACrC,IAAI,MAAoB,CAAC;IACzB,IAAI,CAAC;QACH,MAAM,GAAG,IAAI,CAAC,KAAK,CAAC,cAAc,CAAiB,CAAC;IACtD,CAAC;IAAC,MAAM,CAAC;QACP,oEAAoE;QACpE,MAAM,CAAC,KAAK,CAAC,uCAAuC,CAAC,CAAC;QACtD,MAAM,IAAI,yBAAkB,CAAC,6BAA6B,CAAC,CAAC;IAC9D,CAAC;IAED,iDAAiD;IACjD,IAAI,CAAC,KAAK,CAAC,OAAO,CAAC,MAAM,CAAC,EAAE,CAAC;QAC3B,MAAM,IAAI,yBAAkB,CAC1B,4EAA4E,CAC7E,CAAC;IACJ,CAAC;IAED,MAAM,GAAG,GAAiB,EAAE,CAAC;IAC7B,KAAK,MAAM,CAAC,IAAI,MAAM,EAAE,CAAC;QACvB,mEAAmE;QACnE,IAAI,CAAC,CAAC,QAAQ,KAAK,SAAS,EAAE,CAAC;YAC7B,IAAI,CAAC,SAAS,CAAC,CAAC,CAAC,QAAQ,CAAC,CAAC;QAC7B,CAAC;QACD,IAAI,CAAC,CAAC,KAAK,KAAK,SAAS,EAAE,CAAC;YAC1B,IAAI,CAAC,SAAS,CAAC,CAAC,CAAC,KAAK,CAAC,CAAC;QAC1B,CAAC;QAED,IAAI,CAAC,CAAC,GAAG,KAAK,SAAS,IAAI,CAAC,CAAC,IAAI,KAAK,SAAS,EAAE,CAAC;YAChD,yFAAyF;YACzF,MAAM,IAAI,yBAAkB,CAC1B,gDAAgD,CACjD,CAAC;QACJ,CAAC;QAED,kFAAkF;QAClF,iEAAiE;QACjE,IAAI,uBAAuB,IAAI,CAAC,CAAC,IAAI,KAAK,uBAAuB,EAAE,CAAC;YAClE,SAAS;QACX,CAAC;QAED,MAAM,WAAW,GAAG,CAAC,GAAuB,EAAW,EAAE;YACvD,OAAO,GAAG,CAAC,CAAC,CAAC,gBAAgB,CAAC,IAAI,CAAC,GAAG,CAAC,CAAC,CAAC,CAAC,IAAI,CAAC;QACjD,CAAC,CAAC;QAEF,IACE,CAAC,WAAW,CAAC,CAAC,CAAC,IAAI,CAAC;YACpB,CAAC,WAAW,CAAC,CAAC,CAAC,IAAI,CAAC;YACpB,CAAC,WAAW,CAAC,CAAC,CAAC,GAAG,CAAC;YACnB,CAAC,WAAW,CAAC,CAAC,CAAC,QAAQ,CAAC;YACxB,CAAC,WAAW,CAAC,CAAC,CAAC,QAAQ,CAAC;YACxB,CAAC,WAAW,CAAC,CAAC,CAAC,KAAK,CAAC,EACrB,CAAC;YACD,MAAM,IAAI,yBAAkB,CAC1B,qEAAqE,CACtE,CAAC;QACJ,CAAC;QAED,GAAG,CAAC,IAAI,CAAC;YACP,IAAI,EAAE,CAAC,CAAC,IAAI;YACZ,IAAI,EAAE,CAAC,CAAC,IAAI;YACZ,GAAG,EAAE,CAAC,CAAC,GAAG;YACV,QAAQ,EAAE,CAAC,CAAC,QAAQ;YACpB,QAAQ,EAAE,CAAC,CAAC,QAAQ;YACpB,KAAK,EAAE,CAAC,CAAC,KAAK;SACf,CAAC,CAAC;IACL,CAAC;IACD,OAAO,GAAG,CAAC;AACb,CAAC"}
\ No newline at end of file
+{"version":3,"file":"start-proxy.js","sourceRoot":"","sources":["../src/start-proxy.ts"],"names":[],"mappings":";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;AAgCA,wCA8FC;AA9HD,oDAAsC;AAEtC,2CAAsD;AAEtD,iCAA4C;AAW5C,MAAM,yBAAyB,GAA6B;IAC1D,IAAI,EAAE,kBAAkB;IACxB,MAAM,EAAE,YAAY;IACpB,UAAU,EAAE,cAAc;IAC1B,MAAM,EAAE,cAAc;IACtB,IAAI,EAAE,iBAAiB;IACvB,IAAI,EAAE,gBAAgB;IACtB,EAAE,EAAE,gBAAgB;IACpB,oFAAoF;IACpF,OAAO,EAAE,EAAE;IACX,GAAG,EAAE,EAAE;IACP,KAAK,EAAE,EAAE;CACD,CAAC;AAEX,kEAAkE;AAClE,+DAA+D;AAC/D,gDAAgD;AAChD,SAAgB,cAAc,CAC5B,MAAc,EACd,eAAmC,EACnC,qBAAyC,EACzC,cAAkC;IAElC,MAAM,QAAQ,GAAG,cAAc,CAAC,CAAC,CAAC,IAAA,yBAAa,EAAC,cAAc,CAAC,CAAC,CAAC,CAAC,SAAS,CAAC;IAC5E,MAAM,uBAAuB,GAAG,QAAQ;QACtC,CAAC,CAAC,yBAAyB,CAAC,QAAQ,CAAC;QACrC,CAAC,CAAC,SAAS,CAAC;IAEd,IAAI,cAAsB,CAAC;IAC3B,IAAI,qBAAqB,KAAK,SAAS,EAAE,CAAC;QACxC,MAAM,CAAC,IAAI,CAAC,qCAAqC,CAAC,CAAC;QACnD,cAAc,GAAG,MAAM,CAAC,IAAI,CAAC,qBAAqB,EAAE,QAAQ,CAAC,CAAC,QAAQ,EAAE,CAAC;IAC3E,CAAC;SAAM,IAAI,eAAe,KAAK,SAAS,EAAE,CAAC;QACzC,MAAM,CAAC,IAAI,CAAC,+BAA+B,CAAC,CAAC;QAC7C,cAAc,GAAG,eAAe,CAAC;IACnC,CAAC;SAAM,CAAC;QACN,MAAM,CAAC,IAAI,CAAC,yBAAyB,CAAC,CAAC;QACvC,OAAO,EAAE,CAAC;IACZ,CAAC;IAED,qCAAqC;IACrC,IAAI,MAAoB,CAAC;IACzB,IAAI,CAAC;QACH,MAAM,GAAG,IAAI,CAAC,KAAK,CAAC,cAAc,CAAiB,CAAC;IACtD,CAAC;IAAC,MAAM,CAAC;QACP,oEAAoE;QACpE,MAAM,CAAC,KAAK,CAAC,uCAAuC,CAAC,CAAC;QACtD,MAAM,IAAI,yBAAkB,CAAC,6BAA6B,CAAC,CAAC;IAC9D,CAAC;IAED,iDAAiD;IACjD,IAAI,CAAC,KAAK,CAAC,OAAO,CAAC,MAAM,CAAC,EAAE,CAAC;QAC3B,MAAM,IAAI,yBAAkB,CAC1B,4EAA4E,CAC7E,CAAC;IACJ,CAAC;IAED,MAAM,GAAG,GAAiB,EAAE,CAAC;IAC7B,KAAK,MAAM,CAAC,IAAI,MAAM,EAAE,CAAC;QACvB,IAAI,CAAC,KAAK,IAAI,IAAI,OAAO,CAAC,KAAK,QAAQ,EAAE,CAAC;YACxC,MAAM,IAAI,yBAAkB,CAAC,yCAAyC,CAAC,CAAC;QAC1E,CAAC;QAED,mEAAmE;QACnE,IAAI,CAAC,CAAC,QAAQ,KAAK,SAAS,EAAE,CAAC;YAC7B,IAAI,CAAC,SAAS,CAAC,CAAC,CAAC,QAAQ,CAAC,CAAC;QAC7B,CAAC;QACD,IAAI,CAAC,CAAC,KAAK,KAAK,SAAS,EAAE,CAAC;YAC1B,IAAI,CAAC,SAAS,CAAC,CAAC,CAAC,KAAK,CAAC,CAAC;QAC1B,CAAC;QAED,IAAI,CAAC,CAAC,GAAG,KAAK,SAAS,IAAI,CAAC,CAAC,IAAI,KAAK,SAAS,EAAE,CAAC;YAChD,yFAAyF;YACzF,MAAM,IAAI,yBAAkB,CAC1B,gDAAgD,CACjD,CAAC;QACJ,CAAC;QAED,kFAAkF;QAClF,iEAAiE;QACjE,IAAI,uBAAuB,IAAI,CAAC,CAAC,IAAI,KAAK,uBAAuB,EAAE,CAAC;YAClE,SAAS;QACX,CAAC;QAED,MAAM,WAAW,GAAG,CAAC,GAAuB,EAAW,EAAE;YACvD,OAAO,GAAG,CAAC,CAAC,CAAC,gBAAgB,CAAC,IAAI,CAAC,GAAG,CAAC,CAAC,CAAC,CAAC,IAAI,CAAC;QACjD,CAAC,CAAC;QAEF,IACE,CAAC,WAAW,CAAC,CAAC,CAAC,IAAI,CAAC;YACpB,CAAC,WAAW,CAAC,CAAC,CAAC,IAAI,CAAC;YACpB,CAAC,WAAW,CAAC,CAAC,CAAC,GAAG,CAAC;YACnB,CAAC,WAAW,CAAC,CAAC,CAAC,QAAQ,CAAC;YACxB,CAAC,WAAW,CAAC,CAAC,CAAC,QAAQ,CAAC;YACxB,CAAC,WAAW,CAAC,CAAC,CAAC,KAAK,CAAC,EACrB,CAAC;YACD,MAAM,IAAI,yBAAkB,CAC1B,qEAAqE,CACtE,CAAC;QACJ,CAAC;QAED,GAAG,CAAC,IAAI,CAAC;YACP,IAAI,EAAE,CAAC,CAAC,IAAI;YACZ,IAAI,EAAE,CAAC,CAAC,IAAI;YACZ,GAAG,EAAE,CAAC,CAAC,GAAG;YACV,QAAQ,EAAE,CAAC,CAAC,QAAQ;YACpB,QAAQ,EAAE,CAAC,CAAC,QAAQ;YACpB,KAAK,EAAE,CAAC,CAAC,KAAK;SACf,CAAC,CAAC;IACL,CAAC;IACD,OAAO,GAAG,CAAC;AACb,CAAC"}
\ No newline at end of file
diff --git a/lib/start-proxy.test.js b/lib/start-proxy.test.js
index c41cbd65e..c82975f30 100644
--- a/lib/start-proxy.test.js
+++ b/lib/start-proxy.test.js
@@ -41,6 +41,7 @@ const logging_1 = require("./logging");
 const startProxyExports = __importStar(require("./start-proxy"));
 const testing_utils_1 = require("./testing-utils");
 (0, testing_utils_1.setupTests)(ava_1.default);
+const toEncodedJSON = (data) => Buffer.from(JSON.stringify(data)).toString("base64");
 (0, ava_1.default)("getCredentials prefers registriesCredentials over registrySecrets", async (t) => {
     const registryCredentials = Buffer.from(JSON.stringify([
         { type: "npm_registry", host: "npm.pkg.github.com", token: "abc" },
@@ -58,6 +59,14 @@ const testing_utils_1 = require("./testing-utils");
         message: "Expected credentials data to be an array of configurations, but it is not.",
     });
 });
+(0, ava_1.default)("getCredentials throws error when credential is not an object", async (t) => {
+    const testCredentials = [["foo"], [null]].map(toEncodedJSON);
+    for (const testCredential of testCredentials) {
+        t.throws(() => startProxyExports.getCredentials((0, logging_1.getRunnerLogger)(true), undefined, testCredential, undefined), {
+            message: "Invalid credentials - must be an object",
+        });
+    }
+});
 (0, ava_1.default)("getCredentials throws error when credential missing host and url", async (t) => {
     const registryCredentials = Buffer.from(JSON.stringify([{ type: "npm_registry", token: "abc" }])).toString("base64");
     t.throws(() => startProxyExports.getCredentials((0, logging_1.getRunnerLogger)(true), undefined, registryCredentials, undefined), {
diff --git a/lib/start-proxy.test.js.map b/lib/start-proxy.test.js.map
index de65f1fc4..13af80b18 100644
--- a/lib/start-proxy.test.js.map
+++ b/lib/start-proxy.test.js.map
@@ -1 +1 @@
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\ No newline at end of file
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diff --git a/src/start-proxy.test.ts b/src/start-proxy.test.ts
index 466d528c6..b56345312 100644
--- a/src/start-proxy.test.ts
+++ b/src/start-proxy.test.ts
@@ -6,6 +6,9 @@ import { setupTests } from "./testing-utils";
 
 setupTests(test);
 
+const toEncodedJSON = (data: any) =>
+  Buffer.from(JSON.stringify(data)).toString("base64");
+
 test("getCredentials prefers registriesCredentials over registrySecrets", async (t) => {
   const registryCredentials = Buffer.from(
     JSON.stringify([
@@ -46,6 +49,25 @@ test("getCredentials throws an error when configurations are not an array", asyn
   );
 });
 
+test("getCredentials throws error when credential is not an object", async (t) => {
+  const testCredentials = [["foo"], [null]].map(toEncodedJSON);
+
+  for (const testCredential of testCredentials) {
+    t.throws(
+      () =>
+        startProxyExports.getCredentials(
+          getRunnerLogger(true),
+          undefined,
+          testCredential,
+          undefined,
+        ),
+      {
+        message: "Invalid credentials - must be an object",
+      },
+    );
+  }
+});
+
 test("getCredentials throws error when credential missing host and url", async (t) => {
   const registryCredentials = Buffer.from(
     JSON.stringify([{ type: "npm_registry", token: "abc" }]),
diff --git a/src/start-proxy.ts b/src/start-proxy.ts
index fab666b89..7b7cedaed 100644
--- a/src/start-proxy.ts
+++ b/src/start-proxy.ts
@@ -72,6 +72,10 @@ export function getCredentials(
 
   const out: Credential[] = [];
   for (const e of parsed) {
+    if (e === null || typeof e !== "object") {
+      throw new ConfigurationError("Invalid credentials - must be an object");
+    }
+
     // Mask credentials to reduce chance of accidental leakage in logs.
     if (e.password !== undefined) {
       core.setSecret(e.password);