diff --git a/invoke/terminals.py b/invoke/terminals.py
index 4151ba5e..b54e2283 100644
--- a/invoke/terminals.py
+++ b/invoke/terminals.py
@@ -243,6 +243,7 @@ def bytes_to_read(input_: IO) -> int:
     # it's not a tty but has a fileno, or vice versa; neither is typically
     # going to work re: ioctl().
     if not WINDOWS and isatty(input_) and has_fileno(input_):
-        fionread = fcntl.ioctl(input_, termios.FIONREAD, b"  ")
-        return int(struct.unpack("h", fionread)[0])
+        arg = bytes(bytearray(struct.calcsize("i")))
+        fionread = fcntl.ioctl(input_, termios.FIONREAD, arg)
+        return int(struct.unpack("i", fionread)[0])
     return 1
diff --git a/tests/runners.py b/tests/runners.py
index bad5f15c..c32da5fe 100644
--- a/tests/runners.py
+++ b/tests/runners.py
@@ -1224,7 +1224,7 @@ def fake_ioctl(fd, cmd, buf):
                 # This works since each mocked attr will still be its own mock
                 # object with a distinct 'is' identity.
                 if cmd is termios.FIONREAD:
-                    return struct.pack("h", len(stdin_data))
+                    return struct.pack("i", len(stdin_data))
 
             ioctl.side_effect = fake_ioctl
             # Set up our runner as one w/ mocked stdin writing (simplest way to