diff --git a/internal/shader/expr.go b/internal/shader/expr.go
index 4f5a2548b..892b94c87 100644
--- a/internal/shader/expr.go
+++ b/internal/shader/expr.go
@@ -147,6 +147,7 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr, markLocalVariable
 		var t shaderir.Type
 		switch {
 		case op == shaderir.LessThanOp || op == shaderir.LessThanEqualOp || op == shaderir.GreaterThanOp || op == shaderir.GreaterThanEqualOp || op == shaderir.EqualOp || op == shaderir.NotEqualOp || op == shaderir.AndAnd || op == shaderir.OrOr:
+			// TODO: Check types of the operands.
 			t = shaderir.Type{Main: shaderir.Bool}
 		case lhs[0].Type == shaderir.NumberExpr && rhs[0].Type != shaderir.NumberExpr:
 			if rhst.Main == shaderir.Int {
@@ -168,34 +169,30 @@ func (cs *compileState) parseExpr(block *block, expr ast.Expr, markLocalVariable
 			t = lhst
 		case lhst.Equal(&rhst):
 			t = lhst
-		case lhst.Main == shaderir.Float || lhst.Main == shaderir.Int:
+		case lhst.Main == shaderir.Float:
 			switch rhst.Main {
-			case shaderir.Int:
-				t = lhst
 			case shaderir.Float, shaderir.Vec2, shaderir.Vec3, shaderir.Vec4, shaderir.Mat2, shaderir.Mat3, shaderir.Mat4:
 				t = rhst
 			default:
 				cs.addError(e.Pos(), fmt.Sprintf("types don't match: %s %s %s", lhst.String(), e.Op, rhst.String()))
 				return nil, nil, nil, false
 			}
-		case rhst.Main == shaderir.Float || rhst.Main == shaderir.Int:
+		case rhst.Main == shaderir.Float:
 			switch lhst.Main {
-			case shaderir.Int:
-				t = rhst
 			case shaderir.Float, shaderir.Vec2, shaderir.Vec3, shaderir.Vec4, shaderir.Mat2, shaderir.Mat3, shaderir.Mat4:
 				t = lhst
 			default:
 				cs.addError(e.Pos(), fmt.Sprintf("types don't match: %s %s %s", lhst.String(), e.Op, rhst.String()))
 				return nil, nil, nil, false
 			}
-		case lhst.Main == shaderir.Vec2 && rhst.Main == shaderir.Mat2 ||
-			lhst.Main == shaderir.Mat2 && rhst.Main == shaderir.Vec2:
+		case op == shaderir.Mul && (lhst.Main == shaderir.Vec2 && rhst.Main == shaderir.Mat2 ||
+			lhst.Main == shaderir.Mat2 && rhst.Main == shaderir.Vec2):
 			t = shaderir.Type{Main: shaderir.Vec2}
-		case lhst.Main == shaderir.Vec3 && rhst.Main == shaderir.Mat3 ||
-			lhst.Main == shaderir.Mat3 && rhst.Main == shaderir.Vec3:
+		case op == shaderir.Mul && (lhst.Main == shaderir.Vec3 && rhst.Main == shaderir.Mat3 ||
+			lhst.Main == shaderir.Mat3 && rhst.Main == shaderir.Vec3):
 			t = shaderir.Type{Main: shaderir.Vec3}
-		case lhst.Main == shaderir.Vec4 && rhst.Main == shaderir.Mat4 ||
-			lhst.Main == shaderir.Mat4 && rhst.Main == shaderir.Vec4:
+		case op == shaderir.Mul && (lhst.Main == shaderir.Vec4 && rhst.Main == shaderir.Mat4 ||
+			lhst.Main == shaderir.Mat4 && rhst.Main == shaderir.Vec4):
 			t = shaderir.Type{Main: shaderir.Vec4}
 		default:
 			cs.addError(e.Pos(), fmt.Sprintf("invalid expression: %s %s %s", lhst.String(), e.Op, rhst.String()))
diff --git a/shader_test.go b/shader_test.go
index 802ac80f0..5035ca9fc 100644
--- a/shader_test.go
+++ b/shader_test.go
@@ -15,6 +15,7 @@
 package ebiten_test
 
 import (
+	"fmt"
 	"image"
 	"image/color"
 	"testing"
@@ -1538,3 +1539,63 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		t.Errorf("error must be non-nil but was nil")
 	}
 }
+
+// Issue #1971
+func TestShaderOperatorMultiply(t *testing.T) {
+	cases := []struct {
+		stmt string
+		err  bool
+	}{
+		{stmt: "a := 1 * vec2(2); _ = a", err: false},
+		{stmt: "a := int(1) * vec2(2); _ = a", err: true},
+		{stmt: "a := 1.0 * vec2(2); _ = a", err: false},
+		{stmt: "a := 1 + vec2(2); _ = a", err: false},
+		{stmt: "a := int(1) + vec2(2); _ = a", err: true},
+		{stmt: "a := 1.0 + vec2(2); _ = a", err: false},
+		{stmt: "a := 1 * vec3(2); _ = a", err: false},
+		{stmt: "a := 1.0 * vec3(2); _ = a", err: false},
+		{stmt: "a := 1 * vec4(2); _ = a", err: false},
+		{stmt: "a := 1.0 * vec4(2); _ = a", err: false},
+		{stmt: "a := 1 * mat2(2); _ = a", err: false},
+		{stmt: "a := 1.0 * mat2(2); _ = a", err: false},
+		{stmt: "a := 1 * mat3(2); _ = a", err: false},
+		{stmt: "a := 1.0 * mat3(2); _ = a", err: false},
+		{stmt: "a := 1 * mat4(2); _ = a", err: false},
+		{stmt: "a := 1.0 * mat4(2); _ = a", err: false},
+		{stmt: "a := vec2(1) * 2; _ = a", err: false},
+		{stmt: "a := vec2(1) * 2.0; _ = a", err: false},
+		{stmt: "a := vec2(1) * int(2); _ = a", err: true},
+		{stmt: "a := vec2(1) * vec2(2); _ = a", err: false},
+		{stmt: "a := vec2(1) + vec2(2); _ = a", err: false},
+		{stmt: "a := vec2(1) * vec3(2); _ = a", err: true},
+		{stmt: "a := vec2(1) * vec4(2); _ = a", err: true},
+		{stmt: "a := vec2(1) * mat2(2); _ = a", err: false},
+		{stmt: "a := vec2(1) * mat3(2); _ = a", err: true},
+		{stmt: "a := vec2(1) * mat4(2); _ = a", err: true},
+		{stmt: "a := mat2(1) * 2; _ = a", err: false},
+		{stmt: "a := mat2(1) * 2.0; _ = a", err: false},
+		{stmt: "a := mat2(1) * int(2); _ = a", err: true},
+		{stmt: "a := mat2(1) + 2.0; _ = a", err: false},
+		{stmt: "a := mat2(1) * vec2(2); _ = a", err: false},
+		{stmt: "a := mat2(1) + vec2(2); _ = a", err: true},
+		{stmt: "a := mat2(1) * vec3(2); _ = a", err: true},
+		{stmt: "a := mat2(1) * vec4(2); _ = a", err: true},
+		{stmt: "a := mat2(1) * mat2(2); _ = a", err: false},
+		{stmt: "a := mat2(1) * mat3(2); _ = a", err: true},
+		{stmt: "a := mat2(1) * mat4(2); _ = a", err: true},
+	}
+
+	for _, c := range cases {
+		_, err := ebiten.NewShader([]byte(fmt.Sprintf(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	%s
+	return position
+}`, c.stmt)))
+		if err == nil && c.err {
+			t.Errorf("%s must return an error but does not", c.stmt)
+		} else if err != nil && !c.err {
+			t.Errorf("%s must not return nil but returned %v", c.stmt, err)
+		}
+	}
+}