diff --git a/internal/shader/expr.go b/internal/shader/expr.go
index c9461e03c..fef0ee7cd 100644
--- a/internal/shader/expr.go
+++ b/internal/shader/expr.go
@@ -430,9 +430,6 @@ func (cs *compileState) parseExpr(block *block, fname string, expr ast.Expr, mar
 					return nil, nil, nil, false
 				}
 				t = shaderir.Type{Main: shaderir.Mat4}
-			case shaderir.Smoothstep:
-				// TODO: Check arg types.
-				t = argts[2]
 			case shaderir.Texture2DF:
 				// TODO: Check arg types.
 				t = shaderir.Type{Main: shaderir.Vec4}
@@ -450,7 +447,7 @@ func (cs *compileState) parseExpr(block *block, fname string, expr ast.Expr, mar
 				})
 				return nil, nil, stmts, true
 
-			case shaderir.Clamp, shaderir.Mix:
+			case shaderir.Clamp, shaderir.Mix, shaderir.Smoothstep:
 				// 3 arguments
 				if len(args) != 3 {
 					cs.addError(e.Pos(), fmt.Sprintf("number of %s's arguments must be 3 but %d", callee.BuiltinFunc, len(args)))
@@ -483,9 +480,19 @@ func (cs *compileState) parseExpr(block *block, fname string, expr ast.Expr, mar
 						cs.addError(e.Pos(), fmt.Sprintf("the third arguments for %s must equal to the first/second argument %s or float but %s", callee.BuiltinFunc, argts[0].String(), argts[2].String()))
 						return nil, nil, nil, false
 					}
+				case shaderir.Smoothstep:
+					if (!argts[0].Equal(&argts[1]) || !argts[0].Equal(&argts[2])) && (argts[0].Main != shaderir.Float || argts[1].Main != shaderir.Float) {
+						cs.addError(e.Pos(), fmt.Sprintf("the first and the second arguments for %s must equal to the third argument %s or float but %s and %s", callee.BuiltinFunc, argts[2].String(), argts[0].String(), argts[1].String()))
+						return nil, nil, nil, false
+					}
 				}
 
-				t = argts[0]
+				switch callee.BuiltinFunc {
+				case shaderir.Smoothstep:
+					t = argts[2]
+				default:
+					t = argts[0]
+				}
 
 			case shaderir.Atan2, shaderir.Mod, shaderir.Min, shaderir.Max, shaderir.Step, shaderir.Distance, shaderir.Dot, shaderir.Cross, shaderir.Reflect:
 				// 2 arguments
diff --git a/internal/shader/syntax_test.go b/internal/shader/syntax_test.go
index 64f7989fe..607920a41 100644
--- a/internal/shader/syntax_test.go
+++ b/internal/shader/syntax_test.go
@@ -1944,6 +1944,59 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 	}
 }
 
+// Issue #2184
+func TestSyntaxBuiltinFuncSmoothstepType(t *testing.T) {
+	cases := []struct {
+		stmt string
+		err  bool
+	}{
+		{stmt: "a := smoothstep(); _ = a", err: true},
+		{stmt: "a := smoothstep(1); _ = a", err: true},
+		{stmt: "a := smoothstep(false, false); _ = a", err: true},
+		{stmt: "a := smoothstep(1, 1); _ = a", err: true},
+		{stmt: "a := smoothstep(false, false, false); _ = a", err: true},
+		{stmt: "a := smoothstep(1, 1, 1); _ = a", err: false},
+		{stmt: "a := smoothstep(1.0, 1, 1); _ = a", err: false},
+		{stmt: "a := smoothstep(1, 1.0, 1); _ = a", err: false},
+		{stmt: "a := smoothstep(1, 1, 1.0); _ = a", err: false},
+		{stmt: "a := smoothstep(1, vec2(1), 1); _ = a", err: true},
+		{stmt: "a := smoothstep(1, 1, vec2(1)); _ = a", err: false},
+		{stmt: "a := smoothstep(1, 1, vec3(1)); _ = a", err: false},
+		{stmt: "a := smoothstep(1, 1, vec4(1)); _ = a", err: false},
+		{stmt: "a := smoothstep(1, vec2(1), vec2(1)); _ = a", err: true},
+		{stmt: "a := smoothstep(vec2(1), 1, 1); _ = a", err: true},
+		{stmt: "a := smoothstep(vec2(1), 1, vec2(1)); _ = a", err: true},
+		{stmt: "a := smoothstep(vec2(1), vec2(1), 1); _ = a", err: true},
+		{stmt: "a := smoothstep(vec2(1), vec2(1), vec2(1)); _ = a", err: false},
+		{stmt: "a := smoothstep(vec2(1), vec2(1), vec3(1)); _ = a", err: true},
+		{stmt: "a := smoothstep(vec3(1), 1, 1); _ = a", err: true},
+		{stmt: "a := smoothstep(vec3(1), 1, vec3(1)); _ = a", err: true},
+		{stmt: "a := smoothstep(vec3(1), vec3(1), 1); _ = a", err: true},
+		{stmt: "a := smoothstep(vec3(1), vec3(1), vec3(1)); _ = a", err: false},
+		{stmt: "a := smoothstep(vec4(1), 1, 1); _ = a", err: true},
+		{stmt: "a := smoothstep(vec4(1), 1, vec4(1)); _ = a", err: true},
+		{stmt: "a := smoothstep(vec4(1), vec4(1), 1); _ = a", err: true},
+		{stmt: "a := smoothstep(vec4(1), vec4(1), vec4(1)); _ = a", err: false},
+		{stmt: "a := smoothstep(1, 1, 1, 1); _ = a", err: true},
+	}
+
+	for _, c := range cases {
+		stmt := c.stmt
+		src := fmt.Sprintf(`package main
+
+func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
+	%s
+	return position
+}`, stmt)
+		_, err := compileToIR([]byte(src))
+		if err == nil && c.err {
+			t.Errorf("%s must return an error but does not", stmt)
+		} else if err != nil && !c.err {
+			t.Errorf("%s must not return nil but returned %v", stmt, err)
+		}
+	}
+}
+
 // Issue #2184
 func TestSyntaxBuiltinFuncCrossType(t *testing.T) {
 	cases := []struct {