diff --git a/tools/toolchains/__init__.py b/tools/toolchains/__init__.py
index 4d5562f68f..8741bc80b1 100644
--- a/tools/toolchains/__init__.py
+++ b/tools/toolchains/__init__.py
@@ -863,7 +863,10 @@ class mbedToolchain:
 
         inc_paths = resources.inc_dirs
         if inc_dirs is not None:
-            inc_paths.extend(inc_dirs)
+            if isinstance(inc_dirs, list):
+                inc_paths.extend(inc_dirs)
+            else:
+                inc_paths.append(inc_dirs)
         # De-duplicate include paths
         inc_paths = set(inc_paths)
         # Sort include paths for consistency