diff --git a/pkg/parser/schedule_fuzzy_scatter.go b/pkg/parser/schedule_fuzzy_scatter.go
index 14080e3e916..6471ee1473e 100644
--- a/pkg/parser/schedule_fuzzy_scatter.go
+++ b/pkg/parser/schedule_fuzzy_scatter.go
@@ -168,6 +168,7 @@ func avoidPeakMinutes(hour, minute int) int {
 
 // stableHash returns a deterministic hash value in the range [0, modulo)
 // using FNV-1a hash algorithm, which is stable across platforms and Go versions.
+// If modulo is <= 0, stableHash returns 0 (the range is empty/invalid).
 func stableHash(s string, modulo int) int {
 	h := fnv.New32a()
 	// hash.Hash.Write never returns an error in practice, but check to satisfy gosec G104
@@ -176,7 +177,13 @@ func stableHash(s string, modulo int) int {
 		scheduleFuzzyScatterLog.Printf("Warning: hash write failed: %v", err)
 		return 0
 	}
-	return int(h.Sum32() % uint32(modulo))
+	if modulo <= 0 {
+		scheduleFuzzyScatterLog.Printf("Warning: stableHash called with non-positive modulo %d, returning 0", modulo)
+		return 0
+	}
+	// Use int64 arithmetic to avoid truncation when modulo exceeds math.MaxUint32
+	// on 64-bit platforms where int is 64 bits wide.
+	return int(int64(h.Sum32()) % int64(modulo))
 }
 
 // ScatterSchedule takes a fuzzy cron expression and a workflow identifier
diff --git a/pkg/parser/schedule_fuzzy_scatter_test.go b/pkg/parser/schedule_fuzzy_scatter_test.go
index 73fd341ce25..10055c49325 100644
--- a/pkg/parser/schedule_fuzzy_scatter_test.go
+++ b/pkg/parser/schedule_fuzzy_scatter_test.go
@@ -4,6 +4,7 @@ package parser
 
 import (
 	"fmt"
+	"math"
 	"strings"
 	"testing"
 )
@@ -557,6 +558,23 @@ func TestStableHash(t *testing.T) {
 	if hash1 == hash3 {
 		t.Logf("Warning: different strings produced same hash (rare but possible)")
 	}
+
+	// Test zero modulo returns 0 (guard against divide-by-zero)
+	if got := stableHash("any", 0); got != 0 {
+		t.Errorf("stableHash(_, 0) = %d, want 0", got)
+	}
+
+	// Test negative modulo returns 0 safely
+	if got := stableHash("any", -1); got != 0 {
+		t.Errorf("stableHash(_, -1) = %d, want 0", got)
+	}
+
+	// Test modulo larger than math.MaxUint32 to guard against truncation regression
+	const largeModulo = math.MaxUint32 + 1 // 4_294_967_296
+	hashLarge := stableHash("test-workflow", largeModulo)
+	if hashLarge < 0 || hashLarge >= largeModulo {
+		t.Errorf("stableHash out of range for large modulo: got %d, want [0, %d)", hashLarge, largeModulo)
+	}
 }
 
 func TestScatterScheduleWeekdays(t *testing.T) {