diff --git a/PyIlmBase/PyImath/PyImathUtil.h b/PyIlmBase/PyImath/PyImathUtil.h
index 7a8eb6e7..b70ea742 100644
--- a/PyIlmBase/PyImath/PyImathUtil.h
+++ b/PyIlmBase/PyImath/PyImathUtil.h
@@ -66,7 +66,7 @@ class PyAcquireLock
     PYIMATH_EXPORT PyAcquireLock();
     PYIMATH_EXPORT ~PyAcquireLock();
     PYIMATH_EXPORT PyAcquireLock(const PyAcquireLock& other) = delete;
-    PYIMATH_EXPORT PyAcquireLock & operator = (PyAcquireLock& other) = delete;
+    PYIMATH_EXPORT const PyAcquireLock & operator = (PyAcquireLock& other) = delete;
     
   private:
     PyGILState_STATE _gstate;
@@ -88,7 +88,7 @@ class PyReleaseLock
     PYIMATH_EXPORT PyReleaseLock();
     PYIMATH_EXPORT ~PyReleaseLock();
     PYIMATH_EXPORT PyReleaseLock(const PyReleaseLock& other) = delete;
-    PYIMATH_EXPORT PyReleaseLock & operator = (PyReleaseLock& other) = delete;
+    PYIMATH_EXPORT const PyReleaseLock & operator = (PyReleaseLock& other) = delete;
 
   private:
     PyThreadState *_save;