diff --git a/Source/cmCreateTestSourceList.cxx b/Source/cmCreateTestSourceList.cxx
index 29ff530a96..3b692178d7 100644
--- a/Source/cmCreateTestSourceList.cxx
+++ b/Source/cmCreateTestSourceList.cxx
@@ -93,7 +93,8 @@ bool cmCreateTestSourceList(std::vector<std::string> const& args,
       tests_func_name.end();
     tests_func_name.push_back(func_name);
     if (!already_declared) {
-      forwardDeclareCode += cmStrCat("int ", func_name, "(int, char*[]);\n");
+      forwardDeclareCode +=
+        cmStrCat("extern int ", func_name, "(int, char*[]);\n");
     }
   }