diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8f35b5d7fe32..9f5f6e37a23f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@
 - `[jest-config]` Treat `setupFilesAfterEnv` like `setupFiles` when normalizing configs against presets ([#9495](https://github.com/facebook/jest/pull/9495))
 - `[jest-config]` Support `.mjs` config files on Windows as well ([#9558](https://github.com/facebook/jest/pull/9558))
 - `[jest-cli]` Set `coverageProvider` correctly when provided in config ([#9562](https://github.com/facebook/jest/pull/9562))
+- `[jest-config]` Ensure pattern of `replacePosixSep` is a string ([#9546]https://github.com/facebook/jest/pull/9546)
 - `[jest-matcher-utils]` Fix diff highlight of symbol-keyed object. ([#9499](https://github.com/facebook/jest/pull/9499))
 - `[@jest/reporters]` Notifications should be fire&forget rather than having a timeout ([#9567](https://github.com/facebook/jest/pull/9567))
 - `[jest-resolve]` Fix module identity preservation with symlinks and browser field resolution ([#9511](https://github.com/facebook/jest/pull/9511))
diff --git a/packages/jest-config/src/__tests__/normalize.test.js b/packages/jest-config/src/__tests__/normalize.test.js
index b3b59a894df8..ce238fe2afd8 100644
--- a/packages/jest-config/src/__tests__/normalize.test.js
+++ b/packages/jest-config/src/__tests__/normalize.test.js
@@ -1557,6 +1557,13 @@ describe('testPathPattern', () => {
 
           expect(options.testPathPattern).toBe('a\\\\b|c\\\\d');
         });
+
+        it('coerces all patterns to strings', () => {
+          const argv = {[opt.property]: [1]};
+          const {options} = normalize(initialOptions, argv);
+
+          expect(options.testPathPattern).toBe('1');
+        });
       });
     });
   }
diff --git a/packages/jest-config/src/normalize.ts b/packages/jest-config/src/normalize.ts
index a2454c373732..ea72cdef7f85 100644
--- a/packages/jest-config/src/normalize.ts
+++ b/packages/jest-config/src/normalize.ts
@@ -421,11 +421,13 @@ const buildTestPathPattern = (argv: Config.Argv): string => {
     patterns.push(...argv.testPathPattern);
   }
 
-  const replacePosixSep = (pattern: string) => {
+  const replacePosixSep = (pattern: string | number) => {
+    // yargs coerces positional args into numbers
+    const patternAsString = pattern.toString();
     if (path.sep === '/') {
-      return pattern;
+      return patternAsString;
     }
-    return pattern.replace(/\//g, '\\\\');
+    return patternAsString.replace(/\//g, '\\\\');
   };
 
   const testPathPattern = patterns.map(replacePosixSep).join('|');