diff --git a/src/AbstractDiskWriter.cc b/src/AbstractDiskWriter.cc
index 9e3dac4b..a6191490 100644
--- a/src/AbstractDiskWriter.cc
+++ b/src/AbstractDiskWriter.cc
@@ -398,10 +398,14 @@ void AbstractDiskWriter::ensureMmapWrite(size_t len, int64_t offset)
           errNum = GetLastError();
         }
 #else  // !__MINGW32__
-        mapaddr_ = reinterpret_cast<unsigned char*>(mmap(
-            nullptr, filesize, PROT_READ | PROT_WRITE, MAP_SHARED, fd_, 0));
-        if (!mapaddr_) {
+        void * pa = mmap(nullptr, filesize, PROT_READ | PROT_WRITE, MAP_SHARED, fd_, 0);
+
+        if (pa == MAP_FAILED) {
           errNum = errno;
+          mapaddr_ = nullptr;
+        }
+        else {
+          mapaddr_ = reinterpret_cast<unsigned char*>(pa);
         }
 #endif // !__MINGW32__
         if (mapaddr_) {