diff --git a/sql/expression/function/instr_issue3650_test.go b/sql/expression/function/instr_issue3650_test.go
new file mode 100644
index 0000000000..7e0c9ab1b4
--- /dev/null
+++ b/sql/expression/function/instr_issue3650_test.go
@@ -0,0 +1,66 @@
+// Copyright 2020-2021 Dolthub, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package function
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/dolthub/go-mysql-server/sql"
+	"github.com/dolthub/go-mysql-server/sql/expression"
+	"github.com/dolthub/go-mysql-server/sql/types"
+	"github.com/dolthub/go-mysql-server/test"
+)
+
+// TestInstrIssue3650 covers the two defects reported in
+// dolthub/go-mysql-server#3650:
+//  1. INSTR was case-sensitive for nonbinary strings; MySQL's INSTR is
+//     case-insensitive unless one argument is a binary string.
+//  2. A wrapped substring argument clobbered the haystack, so INSTR
+//     returned 1 unconditionally for any StringWrapper needle.
+func TestInstrIssue3650(t *testing.T) {
+	f := NewInstr(
+		sql.NewEmptyContext(),
+		expression.NewGetField(0, types.LongText, "str", true),
+		expression.NewGetField(1, types.LongText, "substr", false),
+	)
+
+	testCases := []struct {
+		name     string
+		row      sql.Row
+		expected int
+	}{
+		// Defect 1: case-insensitive for nonbinary strings.
+		{"case-insensitive needle", sql.NewRow("xyza", "A"), 4},
+		{"case-insensitive haystack", sql.NewRow("XYZA", "a"), 4},
+		{"case-insensitive mixed", sql.NewRow("Hello World", "world"), 7},
+		{"case-insensitive no match", sql.NewRow("abc", "Z"), 0},
+		// Defect 2: wrapped substring must NOT overwrite the haystack.
+		{"wrapped substr match", sql.NewRow("foobar", test.NewMockStringWrapper("bar")), 4},
+		{"wrapped substr no match", sql.NewRow("foobar", test.NewMockStringWrapper("xyz")), 0},
+		{"wrapped substr case-insensitive", sql.NewRow("foobar", test.NewMockStringWrapper("BAR")), 4},
+	}
+
+	for _, tt := range testCases {
+		t.Run(tt.name, func(t *testing.T) {
+			require := require.New(t)
+			ctx := sql.NewEmptyContext()
+			v, err := f.Eval(ctx, tt.row)
+			require.NoError(err)
+			require.Equal(int64(tt.expected), v)
+		})
+	}
+}
diff --git a/sql/expression/function/substring.go b/sql/expression/function/substring.go
index 3b933133da..809270e7ac 100644
--- a/sql/expression/function/substring.go
+++ b/sql/expression/function/substring.go
@@ -18,6 +18,7 @@ import (
 	"fmt"
 	"reflect"
 	"strings"
+	"unicode"
 
 	"github.com/dolthub/go-mysql-server/sql"
 	"github.com/dolthub/go-mysql-server/sql/types"
@@ -654,7 +655,7 @@ func (i Instr) Eval(ctx *sql.Context, row sql.Row) (interface{}, error) {
 		if err != nil {
 			return nil, err
 		}
-		text = []rune(s)
+		subtext = []rune(s)
 	case []byte:
 		subtext = []rune(string(substr))
 	case sql.BytesWrapper:
@@ -676,7 +677,7 @@ func findSubsequence(text []rune, subtext []rune) int64 {
 	for i := 0; i <= len(text)-len(subtext); i++ {
 		var j int
 		for j = 0; j < len(subtext); j++ {
-			if text[i+j] != subtext[j] {
+			if !runeEqualFold(text[i+j], subtext[j]) {
 				break
 			}
 		}
@@ -687,6 +688,13 @@ func findSubsequence(text []rune, subtext []rune) int64 {
 	return -1
 }
 
+// runeEqualFold reports whether two runes are equal under simple case folding.
+// MySQL's INSTR is case-insensitive unless one argument is a binary string, so
+// the rune comparison folds case to match that behavior.
+func runeEqualFold(a, b rune) bool {
+	return a == b || unicode.ToLower(a) == unicode.ToLower(b)
+}
+
 // IsNullable implements the Expression interface.
 func (i Instr) IsNullable(ctx *sql.Context) bool {
 	return i.str.IsNullable(ctx) || i.substr.IsNullable(ctx)