diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 1e6102ce1..6982e30aa 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -67,7 +67,11 @@ struct ErrPos {
     {
         line = pos.line;
         column = pos.column;
-        file = pos.file;
+        // is file symbol null?
+        if (pos.file.set())
+            file = pos.file;
+        else
+            file = "";
         return *this;
     }
 
diff --git a/src/libutil/tests/logging.cc b/src/libutil/tests/logging.cc
index 6a6fb4ac3..6a58b9425 100644
--- a/src/libutil/tests/logging.cc
+++ b/src/libutil/tests/logging.cc
@@ -289,4 +289,22 @@ namespace nix {
             "what about this " ANSI_YELLOW "%3%" ANSI_NORMAL " " ANSI_YELLOW "one" ANSI_NORMAL);
 
     }
+
+    /* ----------------------------------------------------------------------------
+     * ErrPos
+     * --------------------------------------------------------------------------*/
+
+    TEST(errpos, invalidPos) {
+
+      // contains an invalid symbol, which we should not dereference!
+      Pos invalid;
+
+      // constructing without access violation.
+      ErrPos ep(invalid);
+    
+      // assignment without access violation.
+      ep = invalid;
+
+    }
+
 }