diff --git a/libmisc/StringHeap.cc b/libmisc/StringHeap.cc
index 6c30bafa7..f87c4fcf8 100644
--- a/libmisc/StringHeap.cc
+++ b/libmisc/StringHeap.cc
@@ -210,3 +210,5 @@ ostream& operator << (ostream&out, perm_string that)
 	    out << that.str();
       return out;
 }
+
+const perm_string empty_perm_string = perm_string::literal("");
diff --git a/libmisc/StringHeap.h b/libmisc/StringHeap.h
index eecbec7b0..5c791e104 100644
--- a/libmisc/StringHeap.h
+++ b/libmisc/StringHeap.h
@@ -52,6 +52,7 @@ class perm_string {
       const char*text_;
 };
 
+extern const perm_string empty_perm_string;
 extern bool operator == (perm_string a, perm_string b);
 extern bool operator == (perm_string a, const char* b);
 extern bool operator != (perm_string a, perm_string b);