diff --git a/godotenv.go b/godotenv.go
index 61b0ebb..8e9fe3f 100644
--- a/godotenv.go
+++ b/godotenv.go
@@ -20,7 +20,6 @@ import (
 	"os"
 	"os/exec"
 	"sort"
-	"strconv"
 	"strings"
 )
 
@@ -159,13 +158,33 @@ func Write(envMap map[string]string, filename string) error {
 	return file.Sync()
 }
 
+// isInt checks if the string may be serialized as a number value, leading
+// "-" symbol is allowed for negative numbers, leading "+" sign is not. The
+// length of the value is not limited.
+func isInt(s string) bool {
+	s = strings.TrimPrefix(s, "-")
+
+	if len(s) == 0 {
+		return false
+	}
+
+	for _, r := range s {
+		if '0' <= r && r <= '9' {
+			continue
+		}
+		return false
+	}
+
+	return true
+}
+
 // Marshal outputs the given environment as a dotenv-formatted environment file.
 // Each line is in the format: KEY="VALUE" where VALUE is backslash-escaped.
 func Marshal(envMap map[string]string) (string, error) {
 	lines := make([]string, 0, len(envMap))
 	for k, v := range envMap {
-		if d, err := strconv.Atoi(v); err == nil {
-			lines = append(lines, fmt.Sprintf(`%s=%d`, k, d))
+		if isInt(v) {
+			lines = append(lines, fmt.Sprintf(`%s=%s`, k, v))
 		} else {
 			lines = append(lines, fmt.Sprintf(`%s="%s"`, k, doubleQuoteEscape(v)))
 		}
diff --git a/godotenv_test.go b/godotenv_test.go
index c6d7e54..db9f7e0 100644
--- a/godotenv_test.go
+++ b/godotenv_test.go
@@ -479,6 +479,30 @@ func TestComments(t *testing.T) {
 	loadEnvAndCompareValues(t, Load, envFileName, expectedValues, noopPresets)
 }
 
+func TestIsInt(t *testing.T) {
+	checkAndCompare := func(s string, expected bool) {
+		if isInt(s) != expected {
+			t.Fail()
+		}
+	}
+
+	// invalid values
+	checkAndCompare("", false)
+	checkAndCompare("+123", false)
+	checkAndCompare("+12a3", false)
+	checkAndCompare("12a3", false)
+	checkAndCompare("abc", false)
+	checkAndCompare("12 3", false)
+	checkAndCompare("-", false)
+	checkAndCompare(" ", false)
+
+	// valid values
+	checkAndCompare("-123", true)
+	checkAndCompare("123", true)
+	checkAndCompare("-922337203685477580868712", true)
+	checkAndCompare("922337203685477580837281", true)
+}
+
 func TestWrite(t *testing.T) {
 	writeAndCompare := func(env string, expected string) {
 		envMap, _ := Unmarshal(env)
@@ -582,42 +606,42 @@ func TestWhitespace(t *testing.T) {
 	}{
 		"Leading whitespace": {
 			input: " A=a\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Leading tab": {
 			input: "\tA=a\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Leading mixed whitespace": {
 			input: " \t \t\n\t \t A=a\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Leading whitespace before export": {
 			input: " \t\t export    A=a\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Trailing whitespace": {
 			input: "A=a \t \t\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Trailing whitespace with export": {
 			input: "export A=a\t \t \n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"No EOL": {
 			input: "A=a",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Trailing whitespace with no EOL": {
 			input: "A=a ",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 	}