diff --git a/rugnux/ModelValidation.cpp b/rugnux/ModelValidation.cpp
index 107adc2e3..6c2e02526 100644
--- a/rugnux/ModelValidation.cpp
+++ b/rugnux/ModelValidation.cpp
@@ -21,6 +21,7 @@
 #include <gemmi/ccp4.hpp>      // Ccp4 map I/O
 #include <gemmi/mtz.hpp>       // Mtz (map-coefficient output)
 
+#include "../common/JFJochMath.h"   // PI (M_PI is not standard, and MSVC does not define it)
 #include "../common/Logger.h"
 #include "../image_analysis/scale_merge/ReindexAmbiguity.h"   // ReindexReflections
 
@@ -348,7 +349,7 @@ ModelValidationResult ValidateAgainstModel(const std::vector<MergedReflection> &
             const auto it = danom_by_hkl.find(hkl_key(hv.hkl));
             if (it == danom_by_hkl.end())
                 continue;
-            const auto phi = static_cast<float>(std::arg(hv.value) - M_PI / 2);
+            const auto phi = static_cast<float>(std::arg(hv.value) - PI / 2);
             mapanom.v.push_back({hv.hkl, it->second * std::polar(1.0f, phi)});
         }
         result.anomalous_pairs = static_cast<int>(mapanom.v.size());