Index: arith.c
===================================================================
--- arith.c	(Revision 264897)
+++ arith.c	(Arbeitskopie)
@@ -113,6 +113,11 @@ gfc_arith_error (arith code)
       p =
 	_("Integer outside symmetric range implied by Standard Fortran at %L");
       break;
+    case ARITH_WRONGCONCAT:
+      p =
+	_("Illegal type in character concatenation at %L");
+      break;
+
     default:
       gfc_internal_error ("gfc_arith_error(): Bad error code");
     }
@@ -982,7 +987,12 @@ gfc_arith_concat (gfc_expr *op1, gfc_expr *op2, gf
   gfc_expr *result;
   size_t len;
 
-  gcc_assert (op1->ts.kind == op2->ts.kind);
+  /* By cleverly playing around with constructors, is is possible
+     to get mismaching types here.  */
+  if (op1->ts.type != BT_CHARACTER || op2->ts.type != BT_CHARACTER
+      || op1->ts.kind != op2->ts.kind)
+    return ARITH_WRONGCONCAT;
+
   result = gfc_get_constant_expr (BT_CHARACTER, op1->ts.kind,
 				  &op1->where);
 
Index: gfortran.h
===================================================================
--- gfortran.h	(Revision 264897)
+++ gfortran.h	(Arbeitskopie)
@@ -191,7 +191,8 @@ enum gfc_intrinsic_op
 /* Arithmetic results.  */
 enum arith
 { ARITH_OK = 1, ARITH_OVERFLOW, ARITH_UNDERFLOW, ARITH_NAN,
-  ARITH_DIV0, ARITH_INCOMMENSURATE, ARITH_ASYMMETRIC, ARITH_PROHIBIT
+  ARITH_DIV0, ARITH_INCOMMENSURATE, ARITH_ASYMMETRIC, ARITH_PROHIBIT,
+  ARITH_WRONGCONCAT
 };
 
 /* Statements.  */