diff --git a/include/wx/longlong.h b/include/wx/longlong.h
index cd6b752d1e..f248bb6d21 100644
--- a/include/wx/longlong.h
+++ b/include/wx/longlong.h
@@ -411,7 +411,16 @@ public:
     }
 
         // convert to double
+        //
+        // For some completely obscure reasons compiling the cast below with
+        // VC6 in DLL builds only (!) results in "error C2520: conversion from
+        // unsigned __int64 to double not implemented, use signed __int64" so
+        // we must use a different version for that compiler.
+#ifdef __VISUALC6__
+    double ToDouble() const;
+#else
     double ToDouble() const { return wx_truncate_cast(double, m_ll); }
+#endif
 
     // operations
         // addition
diff --git a/src/common/longlong.cpp b/src/common/longlong.cpp
index 27e771096d..1ceefe0123 100644
--- a/src/common/longlong.cpp
+++ b/src/common/longlong.cpp
@@ -133,6 +133,22 @@ wxULongLongNative& wxULongLongNative::operator=(const class wxULongLongWx &ll)
 }
 #endif
 
+#ifdef __VISUALC6__
+double wxULongLongNative::ToDouble() const
+{
+    // Work around the problem of casting unsigned __int64 to double in VC6
+    // (which for unknown reasons only manifests itself in DLL builds, i.e.
+    // when using /MD).
+    static const __int64 int64_t_max = 9223372036854775807i64;
+    if ( m_ll <= int64_t_max )
+        return wx_truncate_cast(double, (wxLongLong_t)m_ll);
+
+    double d = wx_truncate_cast(double, int64_t_max);
+    d += (__int64)(m_ll - int64_t_max - 1); // The cast is safe because of -1
+    return d + 1;
+}
+#endif // __VISUALC6__
+
 #endif // wxUSE_LONGLONG_NATIVE
 
 // ============================================================================