diff --git a/clang/lib/AST/ByteCode/Context.cpp b/clang/lib/AST/ByteCode/Context.cpp
index cfda6e8ded760..8860bcc54a9c1 100644
--- a/clang/lib/AST/ByteCode/Context.cpp
+++ b/clang/lib/AST/ByteCode/Context.cpp
@@ -245,6 +245,9 @@ bool Context::evaluateStrlen(State &Parent, const Expr *E, uint64_t &Result) {
     if (!FieldDesc->isPrimitiveArray())
       return false;
 
+    if (Ptr.isDummy() || Ptr.isUnknownSizeArray())
+      return false;
+
     unsigned N = Ptr.getNumElems();
     if (Ptr.elemSize() == 1) {
       Result = strnlen(reinterpret_cast<const char *>(Ptr.getRawAddress()), N);
diff --git a/clang/test/AST/ByteCode/strlen-unknown-size-array.cpp b/clang/test/AST/ByteCode/strlen-unknown-size-array.cpp
new file mode 100644
index 0000000000000..ddc857009f57d
--- /dev/null
+++ b/clang/test/AST/ByteCode/strlen-unknown-size-array.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -std=c++20 -fexperimental-new-constant-interpreter %s -verify
+// RUN: %clang_cc1 -std=c++20 %s -verify=ref
+
+// expected-no-diagnostics
+// ref-no-diagnostics
+
+/// Test that __builtin_strlen() on external/unknown declarations doesn't crash the bytecode interpreter.
+extern const char s[];
+void foo(char *x)
+{
+    unsigned long len = __builtin_strlen(s);
+    __builtin_strcpy(x, s);
+}