diff --git a/src/backend/parser/parse_coerce.c b/src/backend/parser/parse_coerce.c
index d3240f4b265..83ed74a22a4 100644
--- a/src/backend/parser/parse_coerce.c
+++ b/src/backend/parser/parse_coerce.c
@@ -540,6 +540,22 @@ coerce_type(ParseState *pstate, Node *node,
 		r->location = location;
 		return (Node *) r;
 	}
+
+	/*
+	 * Callers assume that any input of type unknown is a literal, which can
+	 * be coerced to anything (see can_coerce_type()), and literals were
+	 * handled above.  An unknown-typed expression that is not a literal, such
+	 * as an explicit cast to unknown or a call to unknownin(), gets here, so
+	 * report that as a user error rather than an internal one.
+	 */
+	if (inputTypeId == UNKNOWNOID)
+		ereport(ERROR,
+				(errcode(ERRCODE_CANNOT_COERCE),
+				 errmsg("cannot cast type %s to %s",
+						format_type_be(inputTypeId),
+						format_type_be(targetTypeId)),
+				 parser_coercion_errposition(pstate, location, node)));
+
 	/* If we get here, caller blew it */
 	elog(ERROR, "failed to find conversion function from %s to %s",
 		 format_type_be(inputTypeId), format_type_be(targetTypeId));