diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c
index 8bd88b0..52f646f 100644
--- a/src/backend/parser/parse_func.c
+++ b/src/backend/parser/parse_func.c
@@ -539,6 +539,20 @@ func_match_argtypes(int nargs,
 	FuncCandidateList current_candidate;
 	FuncCandidateList next_candidate;
 	int			ncandidates = 0;
+	CoercionContext	ccontext = COERCION_IMPLICIT;
+
+	/*
+	 * Normally, we allow only implicit casts to be used when deciding which
+	 * function signatures a particular call might match.  Any other behavior
+	 * risks allowing users to inadvertently call the wrong function.
+	 * However, when there is only one candidate, the risk of confusion is
+	 * minimal.  So, in that case, we attempt to match the user's call to the
+	 * lone candidate using either implicit or assignment casts, so that users
+	 * needn't insert explicit casts into the call to match the only available
+	 * candidate.
+	 */
+	if (raw_candidates != NULL && raw_candidates->next == NULL)
+		ccontext = COERCION_ASSIGNMENT;
 
 	*candidates = NULL;
 
@@ -548,7 +562,7 @@ func_match_argtypes(int nargs,
 	{
 		next_candidate = current_candidate->next;
 		if (can_coerce_type(nargs, input_typeids, current_candidate->args,
-							COERCION_IMPLICIT))
+							ccontext))
 		{
 			current_candidate->next = *candidates;
 			*candidates = current_candidate;
@@ -1356,7 +1370,7 @@ make_fn_arguments(ParseState *pstate,
 								   (Node *) na->arg,
 								   actual_arg_types[i],
 								   declared_arg_types[i], -1,
-								   COERCION_IMPLICIT,
+								   COERCION_ASSIGNMENT,
 								   COERCE_IMPLICIT_CAST,
 								   -1);
 				na->arg = (Expr *) node;
@@ -1367,7 +1381,7 @@ make_fn_arguments(ParseState *pstate,
 								   node,
 								   actual_arg_types[i],
 								   declared_arg_types[i], -1,
-								   COERCION_IMPLICIT,
+								   COERCION_ASSIGNMENT,
 								   COERCE_IMPLICIT_CAST,
 								   -1);
 				lfirst(current_fargs) = node;
diff --git a/src/test/regress/expected/errors.out b/src/test/regress/expected/errors.out
index fa0bd82..00be865 100644
--- a/src/test/regress/expected/errors.out
+++ b/src/test/regress/expected/errors.out
@@ -126,7 +126,7 @@ create aggregate newavg2 (sfunc = int4pl,
 			  stype = int4,
 			  finalfunc = int2um,
 			  initcond = '0');
-ERROR:  function int2um(integer) does not exist
+ERROR:  function int2um(smallint) requires run-time type coercion
 -- left out basetype
 create aggregate newcnt1 (sfunc = int4inc,
 			  stype = int4,