diff --git a/src/humanize/filesize.py b/src/humanize/filesize.py
index 996506e..9bb1a62 100644
--- a/src/humanize/filesize.py
+++ b/src/humanize/filesize.py
@@ -37,7 +37,7 @@ def naturalsize(
     gnu: bool = False,
     format: str = "%.1f",
 ) -> str:
-    """Format a number of bytes like a human readable filesize (e.g. 10 kB).
+    """Format a number of bytes like a human-readable filesize (e.g. 10 kB).
 
     By default, decimal suffixes (kB, MB) are used.
 
@@ -83,7 +83,11 @@ def naturalsize(
         suffix = suffixes["decimal"]
 
     base = 1024 if (gnu or binary) else 1000
-    bytes_ = float(value)
+    if isinstance(value, str):
+        bytes_ = float(value)
+    else:
+        bytes_ = value
+
     abs_bytes = abs(bytes_)
 
     if abs_bytes == 1 and not gnu:
diff --git a/tests/test_filesize.py b/tests/test_filesize.py
index 416cbf1..fbb662a 100644
--- a/tests/test_filesize.py
+++ b/tests/test_filesize.py
@@ -12,6 +12,17 @@
     "test_args, expected",
     [
         ([300], "300 Bytes"),
+        (["1000"], "1.0 kB"),
+        ([10**3], "1.0 kB"),
+        ([10**6], "1.0 MB"),
+        ([10**9], "1.0 GB"),
+        ([10**12], "1.0 TB"),
+        ([10**15], "1.0 PB"),
+        ([10**18], "1.0 EB"),
+        ([10**21], "1.0 ZB"),
+        ([10**24], "1.0 YB"),
+        ([10**27], "1.0 RB"),
+        ([10**30], "1.0 QB"),
         ([1000**1 * 31], "31.0 kB"),
         ([1000**2 * 32], "32.0 MB"),
         ([1000**3 * 33], "33.0 GB"),
@@ -67,6 +78,10 @@
 def test_naturalsize(test_args: list[int] | list[int | bool], expected: str) -> None:
     assert humanize.naturalsize(*test_args) == expected
 
-    args_with_negative = test_args
-    args_with_negative[0] *= -1
-    assert humanize.naturalsize(*args_with_negative) == "-" + expected
+    # Retest with negative input
+    if isinstance(test_args[0], int):
+        test_args[0] *= -1
+    else:
+        test_args[0] = f"-{test_args[0]}"
+
+    assert humanize.naturalsize(*test_args) == "-" + expected