diff --git a/layout.go b/layout.go
index c3008e2..b761de8 100644
--- a/layout.go
+++ b/layout.go
@@ -249,3 +249,16 @@ func (e *layoutVariantError) InvalidSlots() []byte {
 	}
 	return append([]byte(nil), e.invalidSlots...)
 }
+
+// InvalidPositions returns a copy of the 1-based positions of the invalid slot
+// characters in the original variant string.
+func (e *layoutVariantError) InvalidPositions() []int {
+	if e == nil || len(e.invalidPositions) == 0 {
+		return nil
+	}
+	positions := make([]int, len(e.invalidPositions))
+	for i, pos := range e.invalidPositions {
+		positions[i] = pos + 1
+	}
+	return positions
+}
diff --git a/layout_test.go b/layout_test.go
index 1db057a..7702686 100644
--- a/layout_test.go
+++ b/layout_test.go
@@ -183,6 +183,13 @@ func TestLayout_VariantError(t *testing.T) {
 			wantRender: `<header role="banner" data-block="H-0">header</header>` +
 				`<main role="main" data-block="C-0">main</main>`,
 		},
+		{
+			name:          "multiple invalid slots",
+			variant:       "H1X?",
+			wantErr:       true,
+			wantErrString: "html: invalid layout variant H1X? (invalid slots: '1' at position 2, 'X' at position 3, '?' at position 4)",
+			wantRender: `<header role="banner" data-block="H-0">header</header>`,
+		},
 	}
 
 	for _, tt := range tests {
@@ -201,8 +208,21 @@ func TestLayout_VariantError(t *testing.T) {
 					t.Fatalf("VariantError().Error() = %q, want %q", got, tt.wantErrString)
 				}
 				if err, ok := layout.VariantError().(*layoutVariantError); ok {
-					if got := string(err.InvalidSlots()); got != "X" {
-						t.Fatalf("InvalidSlots() = %q, want %q", got, "X")
+					switch tt.variant {
+					case "HXC":
+						if got := string(err.InvalidSlots()); got != "X" {
+							t.Fatalf("InvalidSlots() = %q, want %q", got, "X")
+						}
+						if got := err.InvalidPositions(); len(got) != 1 || got[0] != 2 {
+							t.Fatalf("InvalidPositions() = %v, want %v", got, []int{2})
+						}
+					case "H1X?":
+						if got := string(err.InvalidSlots()); got != "1X?" {
+							t.Fatalf("InvalidSlots() = %q, want %q", got, "1X?")
+						}
+						if got := err.InvalidPositions(); len(got) != 3 || got[0] != 2 || got[1] != 3 || got[2] != 4 {
+							t.Fatalf("InvalidPositions() = %v, want %v", got, []int{2, 3, 4})
+						}
 					}
 				} else {
 					t.Fatalf("VariantError() has unexpected concrete type %T", layout.VariantError())