diff --git a/metobs/data/__init__.py b/metobs/data/__init__.py
index 0b76ebd12c8775db532e10be08d09e9112c9f7d6..6b93d1e0cc88825bfdb06291ba1c591818e60752 100644
--- a/metobs/data/__init__.py
+++ b/metobs/data/__init__.py
@@ -313,7 +313,7 @@ def array_convert(arr, sunits, cunits):
     :type cunits: string
     """
     if sunits == cunits: return arr
-    if type(arr) != ndarray: raise ValueError("Array must be of type numpy.ndarray")
+    if not isinstance(arr, ndarray):: raise ValueError("Array must be of type numpy.ndarray, not %r" % (type(arr),))
     shape = arr.shape
     a = arr.flatten()
     if sunits == 'c':