diff --git a/runtime/src/main/kotlin/kotlin/text/StringNumberConversions.kt b/runtime/src/main/kotlin/kotlin/text/StringNumberConversions.kt
index 91f483aab70..b11c98be1ab 100644
--- a/runtime/src/main/kotlin/kotlin/text/StringNumberConversions.kt
+++ b/runtime/src/main/kotlin/kotlin/text/StringNumberConversions.kt
@@ -52,10 +52,11 @@ external internal fun longToString(value: Long, radix: Int): String
 public actual inline fun Long.toString(radix: Int): String = longToString(this, checkRadix(radix))
 
 /**
- * Returns `true` if the contents of this string is equal to the word "true", ignoring case, and `false` otherwise.
+ * Returns `true` if this string is not `null` and its content is equal to the word "true", ignoring case, and `false` otherwise.
  */
+@SinceKotlin("1.4")
 @kotlin.internal.InlineOnly
-public actual inline fun String.toBoolean(): Boolean = this.equals("true", ignoreCase = true)
+public actual inline fun String?.toBoolean(): Boolean = this.equals("true", ignoreCase = true)
 
 /**
  * Parses the string as a signed [Byte] number and returns the result.