diff --git a/strix/interface/main.py b/strix/interface/main.py
index 1334a453..ebfe98f7 100644
--- a/strix/interface/main.py
+++ b/strix/interface/main.py
@@ -333,6 +333,12 @@ def _parse_credentials(
             )
             return result  # unreachable
         str_values: dict[str, str] = {}
+        for k, v in loaded.items():
+            if not isinstance(v, str):
+                parser.error(
+                    f"Credentials file values must be strings, "
+                    f"got {type(v).__name__} for key '{k}': '{credentials_file}'"
+        str_values: dict[str, str] = {}
         for k, v in loaded.items():
             if not isinstance(v, str):
                 parser.error(