diff --git a/src/arachne.c b/src/arachne.c
index f72a88e..473cb2c 100644
--- a/src/arachne.c
+++ b/src/arachne.c
@@ -175,7 +175,7 @@ int
 isTermFunctionName (Term t)
 {
   t = deVar (t);
-  if (t != NULL && isTermLeaf(t) && inTermlist (t->stype, TERM_Function))
+  if (t != NULL && isTermLeaf(t) && t->stype != NULL && inTermlist (t->stype, TERM_Function))
     return 1;
   return 0;
 }
@@ -1470,6 +1470,10 @@ prune_theorems ()
       Term agent;
 
       agent = deVar (tl->term);
+      if (agent == NULL)
+	{
+	  error ("Agent of run 0 is NULL");
+	}
       if (!realTermLeaf (agent))
 	{
 	  if (sys->output == PROOF)
@@ -1511,16 +1515,38 @@ prune_theorems ()
 	{
 	  if (sys->runs[run].protocol != INTRUDER)
 	    {
-	      Term actor;
-
-	      actor = agentOfRun(sys, run);
-	      if (inTermlist (sys->untrusted, actor))
+	      if (sys->runs[run].agents != NULL)
 		{
-		  if (sys->output == PROOF)
+	          Term actor;
+
+		  actor = agentOfRun(sys, run);
+		  if (actor == NULL)
 		    {
-		      indentPrint ();
-		      eprintf ("Pruned because the actor of run %i is untrusted.\n", run);
+		      error ("Agent of run %i is NULL", run);
 		    }
+		  if (inTermlist (sys->untrusted, actor))
+		    {
+		      if (sys->output == PROOF)
+			{
+			  indentPrint ();
+			  eprintf ("Pruned because the actor of run %i is untrusted.\n", run);
+			}
+		    }
+		}
+	      else
+		{
+		  Protocol p;
+
+		  globalError++;
+		  eprintf ("Run %i: ", run);
+                  role_name_print (run);
+		  eprintf (" has an empty agents list.\n");
+		  eprintf ("protocol->rolenames: ");
+		  p = (Protocol) sys->runs[run].protocol;
+		  termlistPrint (p->rolenames);
+		  eprintf ("\n");
+		  error ("Aborting.");
+		  globalError--;
 		}
 	    }
 	  run++;
diff --git a/src/compiler.c b/src/compiler.c
index 25008ab..f3996d4 100644
--- a/src/compiler.c
+++ b/src/compiler.c
@@ -513,6 +513,10 @@ commEvent (int event, Tac tc)
 
       /* store claim in claim list */
 
+      if (label == NULL)
+	{
+	  error ("Claim should have label on line %i.", trip->next->lineno);
+	}
       // First check whether label is unique
       cl = sys->claimlist;
       while (cl != NULL)
diff --git a/src/symbol.c b/src/symbol.c
index 18c375f..42a4b59 100644
--- a/src/symbol.c
+++ b/src/symbol.c
@@ -232,6 +232,12 @@ symbol_fix_keylevels (void)
 	  if (sym->keylevel == INT_MAX)
 	    {
 	      // Nothing currently, this simply does not originate on a strand.
+#ifdef DEBUG
+	      if (DEBUGL (5))
+		{
+		  eprintf (" doesn't have a keylevel yet.\n");
+		}
+#endif
 	    }
 #ifdef DEBUG
 	  else
diff --git a/src/system.c b/src/system.c
index 78e8ca7..98f6d85 100644
--- a/src/system.c
+++ b/src/system.c
@@ -508,7 +508,6 @@ roleInstance (const System sys, const Protocol protocol, const Role role,
   Termlist tolist = NULL;
   Termlist artefacts = NULL;
   Term extterm = NULL;
-  Term newvar;
 
   /* claim runid, allocate space */
   rid = sys->maxruns;
@@ -538,6 +537,8 @@ roleInstance (const System sys, const Protocol protocol, const Role role,
 	  if (scanto->term->stype != NULL &&
 	      inTermlist (scanto->term->stype, TERM_Type))
 	    {
+	      Term newvar;
+
 	      /* There is a TYPE constant in the parameter list.
 	       * Generate a new local variable for this run, with this type */
 	      newvar =