diff --git a/core/common/src/ByteStrings.kt b/core/common/src/ByteStrings.kt
index 8dd269c2..a1468956 100644
--- a/core/common/src/ByteStrings.kt
+++ b/core/common/src/ByteStrings.kt
@@ -10,6 +10,7 @@ import kotlinx.io.bytestring.isEmpty
 import kotlinx.io.bytestring.unsafe.UnsafeByteStringApi
 import kotlinx.io.bytestring.unsafe.UnsafeByteStringOperations
 import kotlinx.io.unsafe.UnsafeBufferOperations
+import kotlin.math.max
 import kotlin.math.min
 
 /**
@@ -85,10 +86,14 @@ public fun Source.readByteString(byteCount: Int): ByteString {
  * expands the source's buffer as necessary until [byteString] is found. This reads an unbounded number of
  * bytes into the buffer. Returns `-1` if the stream is exhausted before the requested bytes are found.
  *
+ * For empty byte strings this function returns [startIndex] if it lays within underlying buffer's bounds,
+ * `0` if [startIndex] was negative and the size of the underlying buffer if [startIndex] exceeds its size.
+ * If the [startIndex] value was greater than the underlying buffer's size, the data will be fetched and buffered
+ * despite the [byteString] is empty.
+ *
  * @param byteString the sequence of bytes to find within the source.
  * @param startIndex the index into the source to start searching from.
  *
- * @throws IllegalArgumentException if [startIndex] is negative.
  * @throws IllegalStateException if the source is closed.
  * @throws IOException when some I/O error occurs.
  *
@@ -96,10 +101,11 @@ public fun Source.readByteString(byteCount: Int): ByteString {
  */
 @OptIn(InternalIoApi::class, UnsafeByteStringApi::class)
 public fun Source.indexOf(byteString: ByteString, startIndex: Long = 0): Long {
-    require(startIndex >= 0) { "startIndex: $startIndex" }
+    val startIndex = max(0, startIndex)
 
     if (byteString.isEmpty()) {
-        return 0
+        request(startIndex)
+        return min(startIndex, buffer.size)
     }
 
     var offset = startIndex
@@ -117,12 +123,22 @@ public fun Source.indexOf(byteString: ByteString, startIndex: Long = 0): Long {
     return -1
 }
 
+/**
+ * Returns the index of the first match for [byteString] in the buffer at or after [startIndex].
+ *
+ * For empty byte strings this function returns [startIndex] if it lays within buffer's bounds,
+ * `0` if [startIndex] was negative and [Buffer.size] if it was greater or equal to [Buffer.size].
+ *
+ * @param byteString the sequence of bytes to find within the buffer.
+ * @param startIndex the index into the buffer to start searching from.
+ *
+ * @sample kotlinx.io.samples.ByteStringSamples.indexOfByteString
+ */
 @OptIn(UnsafeByteStringApi::class)
 public fun Buffer.indexOf(byteString: ByteString, startIndex: Long = 0): Long {
-    require(startIndex <= size) {
-        "startIndex ($startIndex) should not exceed size ($size)"
-    }
-    if (byteString.isEmpty()) return 0
+    val startIndex = max(0, min(startIndex, size))
+
+    if (byteString.isEmpty()) return startIndex
     if (startIndex > size - byteString.size) return -1L
 
     UnsafeByteStringOperations.withByteArrayUnsafe(byteString) { byteStringData ->
diff --git a/core/common/test/AbstractSourceTest.kt b/core/common/test/AbstractSourceTest.kt
index 71347afc..867d6cd3 100644
--- a/core/common/test/AbstractSourceTest.kt
+++ b/core/common/test/AbstractSourceTest.kt
@@ -1723,12 +1723,15 @@ abstract class AbstractBufferedSourceTest internal constructor(
         sink.writeString("flop flip flop")
         sink.emit()
         assertEquals(10, source.indexOf("flop".encodeToByteString(), 1))
+        assertEquals(0, source.indexOf("flop".encodeToByteString(), -1))
         source.readString() // Clear stream
 
-        // Make sure we backtrack and resume searching after partial match.
+        // Make sure we backtrack and resume searching after the partial match.
         sink.writeString("hi hi hi hi hey")
         sink.emit()
         assertEquals(6, source.indexOf("hi hi hey".encodeToByteString(), 1))
+
+        assertEquals(-1, source.indexOf("ho ho ho".encodeToByteString(), 9001))
     }
 
     @Test
@@ -1738,13 +1741,8 @@ abstract class AbstractBufferedSourceTest internal constructor(
         sink.writeString("blablabla")
         sink.emit()
         assertEquals(0, source.indexOf(ByteString()))
-    }
-
-    @Test
-    fun indexOfByteStringInvalidArgumentsThrows() {
-        assertFailsWith<IllegalArgumentException> {
-            source.indexOf("hi".encodeToByteString(), -1)
-        }
+        assertEquals(0, source.indexOf(ByteString(), -1))
+        assertEquals(9, source.indexOf(ByteString(), 100000))
     }
 
     /**
diff --git a/core/common/test/CommonBufferTest.kt b/core/common/test/CommonBufferTest.kt
index d009c5f6..d2b00cbd 100644
--- a/core/common/test/CommonBufferTest.kt
+++ b/core/common/test/CommonBufferTest.kt
@@ -24,6 +24,7 @@ import kotlinx.io.bytestring.ByteString
 import kotlinx.io.bytestring.encodeToByteString
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertFails
 import kotlin.test.assertFailsWith
 import kotlin.test.assertTrue
 
@@ -618,4 +619,24 @@ class CommonBufferTest {
         assertEquals(null, dst.head?.prev)
         assertEquals(null, dst.tail?.next)
     }
+
+    @Test
+    fun indexOfByteString() {
+        val buffer = Buffer()
+        buffer.writeString("hello")
+
+        assertEquals(-1, buffer.indexOf(ByteString(1, 2, 3), -1))
+        assertEquals(-1, buffer.indexOf(ByteString(1, 2, 3), 10))
+
+        assertEquals(2, buffer.indexOf("ll".encodeToByteString()))
+        assertEquals(2, buffer.indexOf("ll".encodeToByteString(), 2))
+        assertEquals(2, buffer.indexOf("ll".encodeToByteString(), -2))
+        assertEquals(-1, buffer.indexOf("ll".encodeToByteString(), 3))
+        assertEquals(-1, buffer.indexOf("hello world".encodeToByteString()))
+
+        assertEquals(0, buffer.indexOf(ByteString()))
+        assertEquals(buffer.size, buffer.indexOf(ByteString(), 1000))
+        assertEquals(1, buffer.indexOf(ByteString(), 1))
+        assertEquals(0, buffer.indexOf(ByteString(), -1))
+    }
 }