diff --git a/contrib/btree_gist/btree_gist.c b/contrib/btree_gist/btree_gist.c
index e1dc253c96..92520aedae 100644
--- a/contrib/btree_gist/btree_gist.c
+++ b/contrib/btree_gist/btree_gist.c
@@ -3,7 +3,7 @@
  */
 #include "postgres.h"
 
-#include "btree_gist.h"
+#include "utils/builtins.h"
 
 PG_MODULE_MAGIC;
 
@@ -19,22 +19,26 @@ PG_FUNCTION_INFO_V1(gbtreekey_out);
 Datum
 gbtreekey_in(PG_FUNCTION_ARGS)
 {
+	Oid			typioparam = PG_GETARG_OID(1);
+
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("<datatype>key_in() not implemented")));
+			 errmsg("cannot accept a value of type %s",
+					format_type_extended(typioparam, -1,
+										 FORMAT_TYPE_ALLOW_INVALID))));
 
-	PG_RETURN_POINTER(NULL);
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
-#include "btree_utils_var.h"
-#include "utils/builtins.h"
 Datum
 gbtreekey_out(PG_FUNCTION_ARGS)
 {
+	/* Sadly, we do not receive any indication of the specific type */
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("<datatype>key_out() not implemented")));
-	PG_RETURN_POINTER(NULL);
+			 errmsg("cannot display a value of type %s", "gbtreekey?")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 
diff --git a/contrib/hstore/hstore_gist.c b/contrib/hstore/hstore_gist.c
index 39ba92a6bf..5d8b806d30 100644
--- a/contrib/hstore/hstore_gist.c
+++ b/contrib/hstore/hstore_gist.c
@@ -96,15 +96,21 @@ PG_FUNCTION_INFO_V1(ghstore_out);
 Datum
 ghstore_in(PG_FUNCTION_ARGS)
 {
-	elog(ERROR, "Not implemented");
-	PG_RETURN_DATUM(0);
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("cannot accept a value of type %s", "ghstore")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 Datum
 ghstore_out(PG_FUNCTION_ARGS)
 {
-	elog(ERROR, "Not implemented");
-	PG_RETURN_DATUM(0);
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("cannot display a value of type %s", "ghstore")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 static GISTTYPE *
diff --git a/contrib/intarray/_intbig_gist.c b/contrib/intarray/_intbig_gist.c
index 887940ca35..dadc18646a 100644
--- a/contrib/intarray/_intbig_gist.c
+++ b/contrib/intarray/_intbig_gist.c
@@ -32,8 +32,9 @@ _intbig_in(PG_FUNCTION_ARGS)
 {
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("_intbig_in() not implemented")));
-	PG_RETURN_DATUM(0);
+			 errmsg("cannot accept a value of type %s", "intbig_gkey")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 Datum
@@ -41,8 +42,9 @@ _intbig_out(PG_FUNCTION_ARGS)
 {
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("_intbig_out() not implemented")));
-	PG_RETURN_DATUM(0);
+			 errmsg("cannot display a value of type %s", "intbig_gkey")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 static GISTTYPE *
diff --git a/contrib/ltree/ltree_gist.c b/contrib/ltree/ltree_gist.c
index b582867a93..5d2db6c62b 100644
--- a/contrib/ltree/ltree_gist.c
+++ b/contrib/ltree/ltree_gist.c
@@ -23,8 +23,9 @@ ltree_gist_in(PG_FUNCTION_ARGS)
 {
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("ltree_gist_in() not implemented")));
-	PG_RETURN_DATUM(0);
+			 errmsg("cannot accept a value of type %s", "ltree_gist")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 Datum
@@ -32,8 +33,9 @@ ltree_gist_out(PG_FUNCTION_ARGS)
 {
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("ltree_gist_out() not implemented")));
-	PG_RETURN_DATUM(0);
+			 errmsg("cannot display a value of type %s", "ltree_gist")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 ltree_gist *
diff --git a/contrib/pg_trgm/trgm_gist.c b/contrib/pg_trgm/trgm_gist.c
index 7a7be807af..3d74a1463a 100644
--- a/contrib/pg_trgm/trgm_gist.c
+++ b/contrib/pg_trgm/trgm_gist.c
@@ -55,15 +55,21 @@ PG_FUNCTION_INFO_V1(gtrgm_options);
 Datum
 gtrgm_in(PG_FUNCTION_ARGS)
 {
-	elog(ERROR, "not implemented");
-	PG_RETURN_DATUM(0);
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("cannot accept a value of type %s", "gtrgm")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 Datum
 gtrgm_out(PG_FUNCTION_ARGS)
 {
-	elog(ERROR, "not implemented");
-	PG_RETURN_DATUM(0);
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("cannot display a value of type %s", "gtrgm")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 static TRGM *
diff --git a/src/backend/utils/adt/tsgistidx.c b/src/backend/utils/adt/tsgistidx.c
index 728b5e9e71..cabfe4d148 100644
--- a/src/backend/utils/adt/tsgistidx.c
+++ b/src/backend/utils/adt/tsgistidx.c
@@ -87,10 +87,12 @@ static int32 sizebitvec(BITVECP sign, int siglen);
 Datum
 gtsvectorin(PG_FUNCTION_ARGS)
 {
+	/* There's no need to support input of gtsvectors */
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-			 errmsg("gtsvector_in not implemented")));
-	PG_RETURN_DATUM(0);
+			 errmsg("cannot accept a value of type %s", "gtsvector")));
+
+	PG_RETURN_VOID();			/* keep compiler quiet */
 }
 
 #define SINGOUTSTR	"%d true bits, %d false bits"