diff --git a/conn.go b/conn.go
index bdb229a9..6c6998b5 100644
--- a/conn.go
+++ b/conn.go
@@ -976,32 +976,12 @@ func (c *Conn) sendPreparedQuery(ps *PreparedStatement, arguments ...interface{}
 
 	wbuf.WriteInt16(int16(len(ps.ParameterOids)))
 	for i, oid := range ps.ParameterOids {
-		switch arguments[i].(type) {
-		case pgtype.BinaryEncoder:
-			wbuf.WriteInt16(BinaryFormatCode)
-		case pgtype.TextEncoder:
-			wbuf.WriteInt16(TextFormatCode)
-		case string, *string:
-			wbuf.WriteInt16(TextFormatCode)
-		default:
-			if dt, ok := c.ConnInfo.DataTypeForOid(oid); ok {
-				switch dt.Value.(type) {
-				case pgtype.BinaryEncoder:
-					wbuf.WriteInt16(BinaryFormatCode)
-				case pgtype.TextEncoder:
-					wbuf.WriteInt16(TextFormatCode)
-				default:
-					return fmt.Errorf("value for oid %v does not implement pgtype.BinaryEncoder or pgtype.TextEncoder", oid)
-				}
-			} else {
-				return fmt.Errorf("unknown type for oid %v", oid)
-			}
-		}
+		wbuf.WriteInt16(chooseParameterFormatCode(c.ConnInfo, oid, arguments[i]))
 	}
 
 	wbuf.WriteInt16(int16(len(arguments)))
 	for i, oid := range ps.ParameterOids {
-		if err := Encode(wbuf, oid, arguments[i]); err != nil {
+		if err := encodePreparedStatementArgument(wbuf, oid, arguments[i]); err != nil {
 			return err
 		}
 	}
diff --git a/copy_from.go b/copy_from.go
index 1f8a2306..9fc76a7b 100644
--- a/copy_from.go
+++ b/copy_from.go
@@ -157,7 +157,7 @@ func (ct *copyFrom) run() (int, error) {
 
 		wbuf.WriteInt16(int16(len(ct.columnNames)))
 		for i, val := range values {
-			err = Encode(wbuf, ps.FieldDescriptions[i].DataType, val)
+			err = encodePreparedStatementArgument(wbuf, ps.FieldDescriptions[i].DataType, val)
 			if err != nil {
 				ct.cancelCopyIn()
 				return 0, err
diff --git a/values.go b/values.go
index aec3cda7..49df5d89 100644
--- a/values.go
+++ b/values.go
@@ -71,10 +71,7 @@ func (e SerializationError) Error() string {
 	return string(e)
 }
 
-// Encode encodes arg into wbuf as the type oid. This allows implementations
-// of the Encoder interface to delegate the actual work of encoding to the
-// built-in functionality.
-func Encode(wbuf *WriteBuf, oid pgtype.Oid, arg interface{}) error {
+func encodePreparedStatementArgument(wbuf *WriteBuf, oid pgtype.Oid, arg interface{}) error {
 	if arg == nil {
 		wbuf.WriteInt32(-1)
 		return nil
@@ -112,7 +109,7 @@ func Encode(wbuf *WriteBuf, oid pgtype.Oid, arg interface{}) error {
 		if err != nil {
 			return err
 		}
-		return Encode(wbuf, oid, v)
+		return encodePreparedStatementArgument(wbuf, oid, v)
 	case string:
 		return encodeString(wbuf, oid, arg)
 	case []byte:
@@ -127,7 +124,7 @@ func Encode(wbuf *WriteBuf, oid pgtype.Oid, arg interface{}) error {
 			return nil
 		}
 		arg = refVal.Elem().Interface()
-		return Encode(wbuf, oid, arg)
+		return encodePreparedStatementArgument(wbuf, oid, arg)
 	}
 
 	if dt, ok := wbuf.conn.ConnInfo.DataTypeForOid(oid); ok {
@@ -152,11 +149,31 @@ func Encode(wbuf *WriteBuf, oid pgtype.Oid, arg interface{}) error {
 	}
 
 	if strippedArg, ok := stripNamedType(&refVal); ok {
-		return Encode(wbuf, oid, strippedArg)
+		return encodePreparedStatementArgument(wbuf, oid, strippedArg)
 	}
 	return SerializationError(fmt.Sprintf("Cannot encode %T into oid %v - %T must implement Encoder or be converted to a string", arg, oid, arg))
 }
 
+// chooseParameterFormatCode determines the correct format code for an
+// argument to a prepared statement. It defaults to TextFormatCode if no
+// determination can be made.
+func chooseParameterFormatCode(ci *pgtype.ConnInfo, oid pgtype.Oid, arg interface{}) int16 {
+	switch arg.(type) {
+	case pgtype.BinaryEncoder:
+		return BinaryFormatCode
+	case string, *string, pgtype.TextEncoder:
+		return TextFormatCode
+	}
+
+	if dt, ok := ci.DataTypeForOid(oid); ok {
+		if _, ok := dt.Value.(pgtype.BinaryEncoder); ok {
+			return BinaryFormatCode
+		}
+	}
+
+	return TextFormatCode
+}
+
 func stripNamedType(val *reflect.Value) (interface{}, bool) {
 	switch val.Kind() {
 	case reflect.Int: