fix 60 interval issue
diff --git a/cronexpr_parse.go b/cronexpr_parse.go
index aeb8296..be6ef17 100644
--- a/cronexpr_parse.go
+++ b/cronexpr_parse.go
@@ -446,6 +446,9 @@
 			directive.first = desc.min
 			directive.last = desc.max
 			directive.step = atoi(snormal[pairs[2]:pairs[3]])
+			if directive.step < 1 || directive.step > desc.max {
+				return nil, fmt.Errorf("invalid interval %s", snormal)
+			}
 			directives = append(directives, &directive)
 			continue
 		}
@@ -456,6 +459,9 @@
 			directive.first = desc.atoi(snormal[pairs[2]:pairs[3]])
 			directive.last = desc.max
 			directive.step = atoi(snormal[pairs[4]:pairs[5]])
+			if directive.step < 1 || directive.step > desc.max {
+				return nil, fmt.Errorf("invalid interval %s", snormal)
+			}
 			directives = append(directives, &directive)
 			continue
 		}
@@ -466,6 +472,9 @@
 			directive.first = desc.atoi(snormal[pairs[2]:pairs[3]])
 			directive.last = desc.atoi(snormal[pairs[4]:pairs[5]])
 			directive.step = atoi(snormal[pairs[6]:pairs[7]])
+			if directive.step < 1 || directive.step > desc.max {
+				return nil, fmt.Errorf("invalid interval %s", snormal)
+			}
 			directives = append(directives, &directive)
 			continue
 		}
diff --git a/cronexpr_test.go b/cronexpr_test.go
index b729170..f170769 100644
--- a/cronexpr_test.go
+++ b/cronexpr_test.go
@@ -284,6 +284,29 @@
 	}
 }
 
+// Issue: https://github.com/gorhill/cronexpr/issues/16
+func TestInterval_Interval60Issue(t *testing.T){
+	_, err := cronexpr.Parse("*/60 * * * * *")
+	if err == nil {
+		t.Errorf("parsing with interval 60 should return err")
+	}
+
+	_, err = cronexpr.Parse("*/61 * * * * *")
+	if err == nil {
+		t.Errorf("parsing with interval 61 should return err")
+	}
+
+	_, err = cronexpr.Parse("2/60 * * * * *")
+	if err == nil {
+		t.Errorf("parsing with interval 60 should return err")
+	}
+
+	_, err = cronexpr.Parse("2-20/61 * * * * *")
+	if err == nil {
+		t.Errorf("parsing with interval 60 should return err")
+	}
+}
+
 /******************************************************************************/
 
 var benchmarkExpressions = []string{