diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4b5b7f6c..d440cf1f 100755
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 All notable changes to **GSTools** will be documented in this file.
 
+## [Unreleased]
+
+### Bugfixes
+- `vario_estimate` was altering the input field unter certain circumstances [#180](https://github.com/GeoStat-Framework/GSTools/issues/180)
+
+
 ## [1.3.1] - Pure Pink - 2021-06
 
 ### Enhancements
diff --git a/gstools/variogram/variogram.py b/gstools/variogram/variogram.py
index f979e61c..c6d98e9f 100644
--- a/gstools/variogram/variogram.py
+++ b/gstools/variogram/variogram.py
@@ -243,7 +243,7 @@ def vario_estimate(
         bin_centres = (bin_edges[:-1] + bin_edges[1:]) / 2.0
     # allow multiple fields at same positions (ndmin=2: first axis -> field ID)
     # need to convert to ma.array, since list of ma.array is not recognised
-    field = np.ma.array(field, ndmin=2, dtype=np.double)
+    field = np.ma.array(field, ndmin=2, dtype=np.double, copy=True)
     masked = np.ma.is_masked(field) or np.any(mask)
     # catch special case if everything is masked
     if masked and np.all(mask):