diff --git a/src/claim.c b/src/claim.c
index ab796e4..2a41cd5 100644
--- a/src/claim.c
+++ b/src/claim.c
@@ -503,41 +503,48 @@ arachne_runs_agree (const System sys, const Claimlist cl, const Termmap runs)
 	int run;
 
 	run = termmapGet (runs, role);
+	if (run != -1)
+	  {
 #ifdef DEBUG
-	if (run < 0 || run >= sys->maxruns)
-	  {
-	    globalError++;
-	    eprintf ("Run mapping %i out of bounds for role ", run);
-	    termPrint (role);
-	    eprintf (" and label ");
-	    termPrint (label);
-	    eprintf ("\n");
-	    eprintf ("This label has sendrole ");
-	    termPrint (linfo->sendrole);
-	    eprintf (" and readrole ");
-	    termPrint (linfo->readrole);
-	    eprintf ("\n");
-	    globalError--;
-	    error ("Run mapping is out of bounds.");
-	  }
+	    if (run < 0 || run >= sys->maxruns)
+	      {
+		globalError++;
+		eprintf ("Run mapping %i out of bounds for role ", run);
+		termPrint (role);
+		eprintf (" and label ");
+		termPrint (label);
+		eprintf ("\n");
+		eprintf ("This label has sendrole ");
+		termPrint (linfo->sendrole);
+		eprintf (" and readrole ");
+		termPrint (linfo->readrole);
+		eprintf ("\n");
+		globalError--;
+		error ("Run mapping is out of bounds.");
+	      }
 #endif
-	rd = sys->runs[run].start;
-	rd_res = NULL;
-	i = 0;
-	while (i < sys->runs[run].step && rd != NULL)
-	  {
-	    if (isTermEqual (rd->label, label))
+	    rd = sys->runs[run].start;
+	    rd_res = NULL;
+	    i = 0;
+	    while (i < sys->runs[run].step && rd != NULL)
 	      {
-		rd_res = rd;
-		rd = NULL;
+		if (isTermEqual (rd->label, label))
+		  {
+		    rd_res = rd;
+		    rd = NULL;
+		  }
+		else
+		  {
+		    rd = rd->next;
+		  }
+		i++;
 	      }
-	    else
-	      {
-		rd = rd->next;
-	      }
-	    i++;
+	    return rd_res;
+	  }
+	else
+	  {
+	    return NULL;
 	  }
-	return rd_res;
       }
 
       // Main
diff --git a/src/term.c b/src/term.c
index 7440962..0eb1930 100644
--- a/src/term.c
+++ b/src/term.c
@@ -237,6 +237,7 @@ isTermEqualFn (Term term1, Term term2)
 /**
  *@param t Term to be checked for a subterm.
  *@param tsub Subterm.
+ *	Note that if t is non-null and tsub is null, it is a valid subterm.
  *@return True iff tsub is a subterm of t.
  */
 int
@@ -246,15 +247,39 @@ termSubTerm (Term t, Term tsub)
   tsub = deVar (tsub);
 
   if (isTermEqual (t, tsub))
-    return 1;
-  if (realTermLeaf (t))
-    return 0;
-  if (realTermTuple (t))
-    return (termSubTerm (TermOp1 (t), tsub)
-	    || termSubTerm (TermOp2 (t), tsub));
+    {
+      return 1;
+    }
   else
-    return (termSubTerm (TermOp (t), tsub)
-	    || termSubTerm (TermKey (t), tsub));
+    {
+      if (t == NULL)
+	{
+	  return 0;
+	}
+      else
+	{
+	  if (tsub == NULL)
+	    {
+	      return 1;
+	    }
+	  else
+	    {
+	      if (realTermLeaf (t))
+		{
+		  return 0;
+		}
+	      else
+		{
+		  if (realTermTuple (t))
+		    return (termSubTerm (TermOp1 (t), tsub)
+			    || termSubTerm (TermOp2 (t), tsub));
+		  else
+		    return (termSubTerm (TermOp (t), tsub)
+			    || termSubTerm (TermKey (t), tsub));
+		}
+	    }
+	}
+    }
 }
 
 //! See if a term is an interm of another.
@@ -1301,7 +1326,6 @@ termPrintDiff (Term t1, Term t2)
 		    }
 		}
 	    }
-
 	}
     }
 }