diff --git a/changelog/unreleased/issue-3861 b/changelog/unreleased/issue-3861
new file mode 100644
index 000000000..3d399df0b
--- /dev/null
+++ b/changelog/unreleased/issue-3861
@@ -0,0 +1,8 @@
+Bugfix: Yield error on invalid policy to `forget`
+
+The `forget` command previously silently ignored invalid/unsupported
+units in the duration options, such as e.g. `--keep-within-daily 2w`.
+
+Specifying an invalid/unsupported duration unit now results in an error.
+
+https://github.com/restic/restic/issues/3861
diff --git a/internal/restic/duration.go b/internal/restic/duration.go
index ad56bcc81..4c9112a7c 100644
--- a/internal/restic/duration.go
+++ b/internal/restic/duration.go
@@ -106,6 +106,8 @@ func ParseDuration(s string) (Duration, error) {
 			d.Days = num
 		case 'h':
 			d.Hours = num
+		default:
+			return Duration{}, errors.Errorf("invalid unit %q found after number %d", s[0], num)
 		}
 
 		s = s[1:]
diff --git a/internal/restic/duration_test.go b/internal/restic/duration_test.go
index 716c00cc9..b46fc66be 100644
--- a/internal/restic/duration_test.go
+++ b/internal/restic/duration_test.go
@@ -63,23 +63,27 @@ func TestParseDuration(t *testing.T) {
 		input  string
 		d      Duration
 		output string
+		err    bool
 	}{
-		{"9h", Duration{Hours: 9}, "9h"},
-		{"3d", Duration{Days: 3}, "3d"},
-		{"4d2h", Duration{Days: 4, Hours: 2}, "4d2h"},
-		{"7m5d", Duration{Months: 7, Days: 5}, "7m5d"},
-		{"6m4d8h", Duration{Months: 6, Days: 4, Hours: 8}, "6m4d8h"},
-		{"5d7m", Duration{Months: 7, Days: 5}, "7m5d"},
-		{"4h3d9m", Duration{Months: 9, Days: 3, Hours: 4}, "9m3d4h"},
-		{"-7m5d", Duration{Months: -7, Days: 5}, "-7m5d"},
-		{"1y4m-5d-3h", Duration{Years: 1, Months: 4, Days: -5, Hours: -3}, "1y4m-5d-3h"},
-		{"2y7m-5d", Duration{Years: 2, Months: 7, Days: -5}, "2y7m-5d"},
+		{input: "9h", d: Duration{Hours: 9}, output: "9h"},
+		{input: "3d", d: Duration{Days: 3}, output: "3d"},
+		{input: "4d2h", d: Duration{Days: 4, Hours: 2}, output: "4d2h"},
+		{input: "7m5d", d: Duration{Months: 7, Days: 5}, output: "7m5d"},
+		{input: "6m4d8h", d: Duration{Months: 6, Days: 4, Hours: 8}, output: "6m4d8h"},
+		{input: "5d7m", d: Duration{Months: 7, Days: 5}, output: "7m5d"},
+		{input: "4h3d9m", d: Duration{Months: 9, Days: 3, Hours: 4}, output: "9m3d4h"},
+		{input: "-7m5d", d: Duration{Months: -7, Days: 5}, output: "-7m5d"},
+		{input: "1y4m-5d-3h", d: Duration{Years: 1, Months: 4, Days: -5, Hours: -3}, output: "1y4m-5d-3h"},
+		{input: "2y7m-5d", d: Duration{Years: 2, Months: 7, Days: -5}, output: "2y7m-5d"},
+		{input: "2w", err: true},
+		{input: "1y4m3w1d", err: true},
+		{input: "s", err: true},
 	}
 
 	for _, test := range tests {
 		t.Run("", func(t *testing.T) {
 			d, err := ParseDuration(test.input)
-			if err != nil {
+			if err != nil && !test.err {
 				t.Fatal(err)
 			}