diff --git a/src/ptvsd/_vendored/pydevd/pydevd_attach_to_process/add_code_to_python_process.py b/src/ptvsd/_vendored/pydevd/pydevd_attach_to_process/add_code_to_python_process.py
index 159b6a923..d2f8e400b 100644
--- a/src/ptvsd/_vendored/pydevd/pydevd_attach_to_process/add_code_to_python_process.py
+++ b/src/ptvsd/_vendored/pydevd/pydevd_attach_to_process/add_code_to_python_process.py
@@ -251,6 +251,8 @@ def resolve_label(process, label):
     for i in range(max_attempts):
         try:
             address = process.resolve_label(label)
+            if not address:
+                raise AssertionError('%s not resolved.' % (label,))
             return address
         except:
             try: