diff --git a/src/phreeqcpp/common/Utils.cxx b/src/phreeqcpp/common/Utils.cxx
index 44f77b17..6738fa9d 100644
--- a/src/phreeqcpp/common/Utils.cxx
+++ b/src/phreeqcpp/common/Utils.cxx
@@ -17,7 +17,9 @@
 #endif
 
 #if __cplusplus < 201103L // Check if C++ standard is pre-C++11
-#  define nullptr NULL
+#  ifndef nullptr
+#    define nullptr NULL
+#  endif
 #endif
 
 #if defined(PHREEQCI_GUI)