diff --git a/google/genai/_api_client.py b/google/genai/_api_client.py
index e6202815e..a3d180225 100644
--- a/google/genai/_api_client.py
+++ b/google/genai/_api_client.py
@@ -362,10 +362,10 @@ def _iter_response_stream(self) -> Iterator[str]:
           data_buffer = []
         continue
 
-      # In streaming mode, the response of JSON is prefixed with "data: " which
+      # In streaming mode, the response of JSON is prefixed with either "data:" or "data: " which
       # we must strip before parsing.
-      if line.startswith('data: '):
-        data_buffer.append(line[len('data: '):])
+      if line.startswith('data:'):
+        data_buffer.append(line[len('data:'):].lstrip())
         continue
 
       # When API returns an error message, it comes line by line. So we buffer