diff --git a/brainpy/_src/initialize/random_inits.py b/brainpy/_src/initialize/random_inits.py
index d7097666..fbad02dd 100644
--- a/brainpy/_src/initialize/random_inits.py
+++ b/brainpy/_src/initialize/random_inits.py
@@ -83,7 +83,7 @@ def _format_shape(shape):
   if len(shape) == 0:
     raise ValueError('Please provide shape.')
   if len(shape) == 1:
-    if isinstance(shape, (tuple, list)):
+    if isinstance(shape[0], (tuple, list)):
       return shape[0]
     else:
       return shape