diff --git a/musttag.go b/musttag.go
index 1254288..0a4173f 100644
--- a/musttag.go
+++ b/musttag.go
@@ -3,6 +3,7 @@ package musttag
 
 import (
 	"flag"
+	"fmt"
 	"go/ast"
 	"go/token"
 	"go/types"
@@ -81,7 +82,7 @@ func flags(funcs *[]Func) flag.FlagSet {
 
 // for tests only.
 var (
-	reportf = func(pass *analysis.Pass, st *structType, fn Func, fnPos token.Position) {
+	report = func(pass *analysis.Pass, st *structType, fn Func, fnPos token.Position) {
 		const format = "`%s` should be annotated with the `%s` tag as it is passed to `%s` at %s"
 		pass.Reportf(st.Pos, format, st.Name, fn.Tag, fn.shortName(), fnPos)
 	}
@@ -106,6 +107,10 @@ func run(pass *analysis.Pass, funcs map[string]Func) (any, error) {
 	filter := []ast.Node{(*ast.CallExpr)(nil)}
 
 	walk.Preorder(filter, func(n ast.Node) {
+		if err != nil {
+			return // there is already an error.
+		}
+
 		call, ok := n.(*ast.CallExpr)
 		if !ok {
 			return // not a function call.
@@ -122,7 +127,8 @@ func run(pass *analysis.Pass, funcs map[string]Func) (any, error) {
 		}
 
 		if len(call.Args) <= fn.ArgPos {
-			return // TODO(junk1tm): return a proper error.
+			err = fmt.Errorf("Func.ArgPos cannot be %d: %s accepts only %d argument(s)", fn.ArgPos, fn.Name, len(call.Args))
+			return
 		}
 
 		arg := call.Args[fn.ArgPos]
@@ -159,10 +165,10 @@ func run(pass *analysis.Pass, funcs map[string]Func) (any, error) {
 
 		p := pass.Fset.Position(call.Pos())
 		p.Filename, _ = filepath.Rel(moduleDir, p.Filename)
-		reportf(pass, result, fn, p)
+		report(pass, result, fn, p)
 	})
 
-	return nil, nil
+	return nil, err
 }
 
 // structType is an extension for types.Struct.
diff --git a/musttag_test.go b/musttag_test.go
index 0b81bbf..c7ad37f 100644
--- a/musttag_test.go
+++ b/musttag_test.go
@@ -19,22 +19,36 @@ func TestAnalyzer(t *testing.T) {
 	prepareTestFiles(t)
 	testPackages = []string{"tests", "builtins"}
 
-	analyzer := New(
-		Func{Name: "example.com/custom.Marshal", Tag: "custom", ArgPos: 0},
-		Func{Name: "example.com/custom.Unmarshal", Tag: "custom", ArgPos: 1},
-	)
+	testdata := analysistest.TestData()
+
+	t.Run("tests", func(t *testing.T) {
+		r := report
+		defer func() { report = r }()
+		report = func(pass *analysis.Pass, st *structType, fn Func, fnPos token.Position) {
+			pass.Reportf(st.Pos, fn.shortName())
+		}
+		analyzer := New()
+		analysistest.Run(t, testdata, analyzer, "tests")
+	})
 
 	t.Run("builtins", func(t *testing.T) {
-		testdata := analysistest.TestData()
+		analyzer := New(
+			Func{Name: "example.com/custom.Marshal", Tag: "custom", ArgPos: 0},
+			Func{Name: "example.com/custom.Unmarshal", Tag: "custom", ArgPos: 1},
+		)
 		analysistest.Run(t, testdata, analyzer, "builtins")
 	})
 
-	t.Run("tests", func(t *testing.T) {
-		reportf = func(pass *analysis.Pass, st *structType, fn Func, fnPos token.Position) {
-			pass.Reportf(st.Pos, fn.shortName())
+	t.Run("bad Func.ArgPos", func(t *testing.T) {
+		const want = `Func.ArgPos cannot be 10: encoding/json.Marshal accepts only 1 argument(s)`
+		analyzer := New(
+			// override the builtin function.
+			Func{Name: "encoding/json.Marshal", Tag: "json", ArgPos: 10},
+		)
+		result := analysistest.Run(nopT{}, testdata, analyzer, "tests")[0]
+		if got := result.Err.Error(); got != want {
+			t.Errorf("\ngot\t%s\nwant\t%s", got, want)
 		}
-		testdata := analysistest.TestData()
-		analysistest.Run(t, testdata, analyzer, "tests")
 	})
 }
 
@@ -46,7 +60,7 @@ func TestFlags(t *testing.T) {
 	t.Run("ok", func(t *testing.T) {
 		err := analyzer.Flags.Parse([]string{"-fn=test.Test:test:0"})
 		if err != nil {
-			t.Errorf("got %v; want no error", err)
+			t.Errorf("\ngot\t%s\nwant\tno error", err)
 		}
 	})
 
@@ -54,7 +68,7 @@ func TestFlags(t *testing.T) {
 		const want = `invalid value "test.Test" for flag -fn: invalid syntax`
 		err := analyzer.Flags.Parse([]string{"-fn=test.Test"})
 		if got := err.Error(); got != want {
-			t.Errorf("got %q; want %q", got, want)
+			t.Errorf("\ngot\t%s\nwant\t%s", got, want)
 		}
 	})
 
@@ -62,11 +76,15 @@ func TestFlags(t *testing.T) {
 		const want = `invalid value "test.Test:test:-" for flag -fn: strconv.Atoi: parsing "-": invalid syntax`
 		err := analyzer.Flags.Parse([]string{"-fn=test.Test:test:-"})
 		if got := err.Error(); got != want {
-			t.Errorf("got %q; want %q", got, want)
+			t.Errorf("\ngot\t%s\nwant\t%s", got, want)
 		}
 	})
 }
 
+type nopT struct{}
+
+func (nopT) Errorf(string, ...any) {}
+
 func prepareTestFiles(t *testing.T) {
 	testdata := analysistest.TestData()