diff --git a/src/humanize/filesize.py b/src/humanize/filesize.py
index fb675fdc..40ba26b8 100644
--- a/src/humanize/filesize.py
+++ b/src/humanize/filesize.py
@@ -103,8 +103,14 @@ def naturalsize(
     # mantissa afterward; rounding can push it up to `base` (e.g. 999999 is
     # 999.999 kB, which formats to "1000.0 kB"). When that happens and a larger
     # suffix is available, step up one suffix so the result reads "1.0 MB".
-    if exp < len(suffix) and abs(float(format % (abs_bytes / (base**exp)))) >= base:
-        exp += 1
+    if exp < len(suffix):
+        mantissa_text = format % (abs_bytes / (base**exp))
+        try:
+            mantissa = float(mantissa_text)
+        except ValueError:
+            mantissa = None
+        if mantissa is not None and abs(mantissa) >= base:
+            exp += 1
     space = "" if gnu else " "
     ret: str = format % (bytes_ / (base**exp)) + space + _(suffix[exp - 1])
     return ret
diff --git a/tests/test_filesize.py b/tests/test_filesize.py
index e6956399..2e2bd516 100644
--- a/tests/test_filesize.py
+++ b/tests/test_filesize.py
@@ -103,3 +103,14 @@ def test_naturalsize(test_args: list[int] | list[int | bool], expected: str) ->
         test_args[0] = f"-{test_args[0]}"
 
     assert humanize.naturalsize(*test_args) == "-" + expected
+
+
+def test_naturalsize_custom_format_with_text() -> None:
+    # regression: custom format strings with surrounding text must not crash
+    # https://github.com/python-humanize/humanize/issues/366
+    assert (
+        humanize.naturalsize(999_999, gnu=True, format="Size: %.1f") == "Size: 976.6K"
+    )
+    assert (
+        humanize.naturalsize(999_999, gnu=True, format="%.1f bytes") == "976.6 bytesK"
+    )