diff --git a/src/pymor/vectorarrays/numpy.py b/src/pymor/vectorarrays/numpy.py
index 099855f8de..f197ba0338 100644
--- a/src/pymor/vectorarrays/numpy.py
+++ b/src/pymor/vectorarrays/numpy.py
@@ -78,6 +78,8 @@ def make_array(cls, subtype=None, count=0, reserve=0):
 
     @property
     def data(self):
+        if self._refcount[0] > 1:
+            self._deep_copy()
         return self._array[:self._len]
 
     @property