diff --git a/src/runtime/c/pgf/graphviz.cxx b/src/runtime/c/pgf/graphviz.cxx
index fc7e69a4c..4f0aa6e3e 100644
--- a/src/runtime/c/pgf/graphviz.cxx
+++ b/src/runtime/c/pgf/graphviz.cxx
@@ -12,6 +12,7 @@ PgfLinearizationGraphvizOutput::PgfLinearizationGraphvizOutput()
     internals = NULL;
     leaves.n_nodes = 0;
     leaves.nodes = NULL;
+    nonexist = false;
 
     meta = (PgfText*) malloc(sizeof(PgfText)+2);
     meta->text[0] = '?';
@@ -20,6 +21,20 @@ PgfLinearizationGraphvizOutput::PgfLinearizationGraphvizOutput()
 
 PgfLinearizationGraphvizOutput::~PgfLinearizationGraphvizOutput()
 {
+    for (size_t i = 0; i < n_internals; i++) {
+        ParseLevel *internal = internals[i];
+        for (size_t j = 0; j < internal->n_nodes; j++) {
+            delete internal->nodes[j];
+        }
+        delete(internal->nodes);
+        delete[] internal;
+    }
+
+    for (size_t j = 0; j < leaves.n_nodes; j++) {
+        delete leaves.nodes[j];
+    }
+    delete(leaves.nodes);
+
     free(meta);
 }
 
@@ -85,6 +100,7 @@ void PgfLinearizationGraphvizOutput::end_phrase(PgfText *cat, int fid, PgfText *
 
 void PgfLinearizationGraphvizOutput::symbol_ne()
 {
+    nonexist = true;
 }
 
 void PgfLinearizationGraphvizOutput::symbol_bind()
@@ -174,6 +190,9 @@ void PgfLinearizationGraphvizOutput::generate_graphviz_level(PgfPrinter *printer
 
 PgfText *PgfLinearizationGraphvizOutput::generate_graphviz(PgfGraphvizOptions* opts)
 {
+    if (nonexist)
+        return NULL;
+
     PgfPrinter printer(NULL, 0, NULL);
 
 	printer.puts("graph {\n");
diff --git a/src/runtime/c/pgf/graphviz.h b/src/runtime/c/pgf/graphviz.h
index 9d4430d24..425678298 100644
--- a/src/runtime/c/pgf/graphviz.h
+++ b/src/runtime/c/pgf/graphviz.h
@@ -28,6 +28,8 @@ class PGF_INTERNAL_DECL PgfLinearizationGraphvizOutput : public PgfLinearization
 
     PgfText *meta;
 
+    bool nonexist;
+
     void generate_graphviz_level(PgfPrinter *printer, PgfGraphvizOptions* opts, ParseLevel *level);
 
 public:
diff --git a/src/runtime/haskell/PGF2.hsc b/src/runtime/haskell/PGF2.hsc
index 5d4fdd628..3b59e85d2 100644
--- a/src/runtime/haskell/PGF2.hsc
+++ b/src/runtime/haskell/PGF2.hsc
@@ -1179,7 +1179,9 @@ graphvizParseTree c opts e =
   withForeignPtr marshaller $ \m ->
   withGraphvizOptions opts $ \c_opts ->
   bracket (withPgfExn "graphvizParseTree" (pgf_graphviz_parse_tree  (c_db c) c_revision c_e nullPtr m c_opts)) free $ \c_text ->
-    peekText c_text
+    if c_text == nullPtr
+      then return ""
+      else peekText c_text
 
 graphvizWordAlignment :: [Concr] -> GraphvizOptions -> Expr -> String
 graphvizWordAlignment cs opts e = error "TODO: graphvizWordAlignment"