diff --git a/filter/decl.m4 b/filter/decl.m4
index 5242c04c..4f7ca3ad 100644
--- a/filter/decl.m4
+++ b/filter/decl.m4
@@ -238,9 +238,13 @@ m4_define(ERROR,
 #	This macro specifies result type and makes there are no conflicting definitions
 m4_define(RESULT_TYPE,
 	`m4_ifdef([[INST_RESULT_TYPE]],
-		  [[m4_ifelse(INST_RESULT_TYPE,$1,,[[ERROR([[Multiple type definitons]])]])]],
+		  [[m4_ifelse(INST_RESULT_TYPE,$1,,[[ERROR([[Multiple type definitions in]] INST_NAME)]])]],
 		  [[m4_define(INST_RESULT_TYPE,$1) RESULT_TYPE_($1)]])')
 
+m4_define(RESULT_TYPE_CHECK,
+	`m4_ifelse(INST_OUTVAL,0,,
+		   [[m4_ifdef([[INST_RESULT_TYPE]],,[[ERROR([[Missing type definition in]] INST_NAME)]])]])')
+
 m4_define(RESULT_TYPE_, `
 FID_NEW_BODY()m4_dnl
 what->type = $1;
@@ -294,6 +298,7 @@ m4_define(FID_ITERATE, `FID_ZONE(10, Iteration)')
 
 #	This macro does all the code wrapping. See inline comments.
 m4_define(INST_FLUSH, `m4_ifdef([[INST_NAME]], [[
+RESULT_TYPE_CHECK()m4_dnl		 Check for defined RESULT_TYPE()
 FID_ENUM()m4_dnl			 Contents of enum fi_code { ... }
   INST_NAME(),
 FID_ENUM_STR()m4_dnl			 Contents of const char * indexed by enum fi_code
@@ -392,6 +397,7 @@ m4_define(INST, `m4_dnl				This macro is called on beginning of each instruction
 INST_FLUSH()m4_dnl				First, old data is flushed
 m4_define([[INST_NAME]], [[$1]])m4_dnl		Then we store instruction name,
 m4_define([[INST_INVAL]], [[$2]])m4_dnl		instruction input value count,
+m4_define([[INST_OUTVAL]], [[$3]])m4_dnl	instruction output value count,
 m4_undefine([[INST_NEVER_CONSTANT]])m4_dnl	reset NEVER_CONSTANT trigger,
 m4_undefine([[INST_RESULT_TYPE]])m4_dnl		and reset RESULT_TYPE value.
 FID_INTERPRET_BODY()m4_dnl 			By default, every code is interpreter code.
diff --git a/filter/f-inst.c b/filter/f-inst.c
index 2120440d..2a02c8e5 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -1103,7 +1103,7 @@
     }
   }
 
-  INST(FI_RETURN, 1, 1) {
+  INST(FI_RETURN, 1, 0) {
     NEVER_CONSTANT;
     /* Acquire the return value */
     ARG_ANY(1);
@@ -1134,6 +1134,9 @@
     VARARG;
     SYMBOL;
 
+    /* Fake result type declaration */
+    RESULT_TYPE(T_VOID);
+
     FID_NEW_BODY()
     ASSERT(sym->class == SYM_FUNCTION);
 
@@ -1443,7 +1446,7 @@
 
   }
 
-  INST(FI_FORMAT, 1, 0) {	/* Format */
+  INST(FI_FORMAT, 1, 1) {	/* Format */
     ARG_ANY(1);
     RESULT(T_STRING, s, val_format_str(fpool, &v1));
   }