diff --git a/internal/parser/parser.go b/internal/parser/parser.go
index 6151333..ac43c12 100644
--- a/internal/parser/parser.go
+++ b/internal/parser/parser.go
@@ -44,10 +44,10 @@ func (t Task) ToRemind(indent int) string {
 			// k must have the first letter uppercase for Remind
 			kUpper := strings.ToUpper(k[:1]) + k[1:]
 			if len(values) == 1 {
-				sb.WriteString(fmt.Sprintf("INFO \"%s: %s\" ", kUpper, values[0]))
+				fmt.Fprintf(&sb, "INFO \"%s: %s\" ", kUpper, values[0])
 			} else {
 				for i, v := range values {
-					sb.WriteString(fmt.Sprintf("INFO \"%s%d: %s\" ", kUpper, i+1, v))
+					fmt.Fprintf(&sb, "INFO \"%s%d: %s\" ", kUpper, i+1, v)
 				}
 			}
 		}
@@ -55,22 +55,22 @@ func (t Task) ToRemind(indent int) string {
 
 	if len(t.Projects) > 0 {
 		for _, p := range t.Projects {
-			sb.WriteString(fmt.Sprintf("INFO \"List: %s\" ", p))
+			fmt.Fprintf(&sb, "INFO \"List: %s\" ", p)
 		}
 	}
 
 	if len(t.Contexts) > 0 {
 		for _, c := range t.Contexts {
-			sb.WriteString(fmt.Sprintf("INFO \"Tag: %s\" ", c))
+			fmt.Fprintf(&sb, "INFO \"Tag: %s\" ", c)
 		}
 	}
 
 	if t.Completed {
-		sb.WriteString(fmt.Sprintf("COMPLETE-THROUGH %s ", t.DueDate.Format("2006-01-02")))
+		fmt.Fprintf(&sb, "COMPLETE-THROUGH %s ", t.DueDate.Format("2006-01-02"))
 	}
 
 	if t.Priority != nil {
-		sb.WriteString(fmt.Sprintf("PRIORITY %d ", t.PriorityAsRemind()))
+		fmt.Fprintf(&sb, "PRIORITY %d ", t.PriorityAsRemind())
 	}
 
 	sb.WriteString("MSG")
@@ -79,7 +79,7 @@ func (t Task) ToRemind(indent int) string {
 		sb.WriteString(" %<List>:")
 	}
 
-	sb.WriteString(fmt.Sprintf(" %s", t.Description))
+	fmt.Fprintf(&sb, " %s", t.Description)
 
 	if t.Completed {
 		sb.WriteString("%:")
@@ -97,11 +97,11 @@ func (t Task) ToRemind(indent int) string {
 			// uppercase first letter for Remind
 			kUpper := strings.ToUpper(k[:1]) + k[1:]
 			if len(values) == 1 {
-				sb.WriteString(fmt.Sprintf("%%_%s%s: %%<%s>", headingSpaces, kUpper, kUpper))
+				fmt.Fprintf(&sb, "%%_%s%s: %%<%s>", headingSpaces, kUpper, kUpper)
 			} else {
 				for i := range values {
 					kNumbered := fmt.Sprintf("%s%d", kUpper, i+1)
-					sb.WriteString(fmt.Sprintf("%%_%s%s: %%<%s>", headingSpaces, kNumbered, kNumbered))
+					fmt.Fprintf(&sb, "%%_%s%s: %%<%s>", headingSpaces, kNumbered, kNumbered)
 				}
 			}
 		}
@@ -202,6 +202,7 @@ func ParseLine(line string) (Task, error) {
 	working := strings.TrimSpace(line)
 	toks := splitTokens(working)
 	i := 0
+	var invalidDue string
 
 	// 1) Completed?
 	if i < len(toks) && toks[i] == "x" {
@@ -285,9 +286,12 @@ func ParseLine(line string) (Task, error) {
 			k, v := parts[0], parts[1]
 			if k != "" && v != "" && !isProtocolKey(k) {
 				t.Metadata[k] = append(t.Metadata[k], v)
-				if k == "due" && dateRe.MatchString(v) {
-					if dt, err := time.Parse(dateLayout, v); err == nil {
+				if k == "due" {
+					dt, err := time.Parse(dateLayout, v)
+					if dateRe.MatchString(v) && err == nil {
 						t.DueDate = &dt
+					} else {
+						invalidDue = v
 					}
 				}
 				continue
@@ -302,6 +306,10 @@ func ParseLine(line string) (Task, error) {
 		return t, errors.New("completed task missing completion date (spec requires completion date directly after 'x')")
 	}
 
+	if invalidDue != "" {
+		return t, fmt.Errorf("invalid due date %q: expected format YYYY-MM-DD (task will be dropped, no Remind entry generated)", invalidDue)
+	}
+
 	return t, nil
 }
 
diff --git a/test.todo.txt b/test.todo.txt
index 385f81d..1f33379 100644
--- a/test.todo.txt
+++ b/test.todo.txt
@@ -73,6 +73,9 @@ X 2012-01-01 Make resolutions
 (A) 2024-13-99 Call Mom
 x 2024-99-99 Call Mom
 
+# Malformed: due date with invalid format (should error, task dropped from output)
+(A) Call Mom due:2024-13-99
+
 # Malformed: key:value with whitespace in key or value
 foo bar:baz
 foo:bar baz