diff --git a/Lib/aifc.py b/Lib/aifc.py
index d8b7fab..9ac710f 100644
--- a/Lib/aifc.py
+++ b/Lib/aifc.py
@@ -778,7 +778,7 @@
 
     def _ensure_header_written(self, datasize):
         if not self._nframeswritten:
-            if self._comptype in ('ULAW', 'ALAW'):
+            if self._comptype in ('ULAW', 'ulaw', 'ALAW', 'alaw'):
                 if not self._sampwidth:
                     self._sampwidth = 2
                 if self._sampwidth != 2:
@@ -844,7 +844,7 @@
         if self._datalength & 1:
             self._datalength = self._datalength + 1
         if self._aifc:
-            if self._comptype in ('ULAW', 'ALAW'):
+            if self._comptype in ('ULAW', 'ulaw', 'ALAW', 'alaw'):
                 self._datalength = self._datalength // 2
                 if self._datalength & 1:
                     self._datalength = self._datalength + 1
@@ -852,7 +852,10 @@
                 self._datalength = (self._datalength + 3) // 4
                 if self._datalength & 1:
                     self._datalength = self._datalength + 1
-        self._form_length_pos = self._file.tell()
+        try:
+            self._form_length_pos = self._file.tell()
+        except (AttributeError, IOError):
+            self._form_length_pos = None
         commlength = self._write_form_length(self._datalength)
         if self._aifc:
             self._file.write('AIFC')
@@ -864,7 +867,8 @@
         self._file.write('COMM')
         _write_ulong(self._file, commlength)
         _write_short(self._file, self._nchannels)
-        self._nframes_pos = self._file.tell()
+        if self._form_length_pos is not None:
+            self._nframes_pos = self._file.tell()
         _write_ulong(self._file, self._nframes)
         if self._comptype in ('ULAW', 'ulaw', 'ALAW', 'alaw', 'G722'):
             _write_short(self._file, 8)
@@ -875,7 +879,8 @@
             self._file.write(self._comptype)
             _write_string(self._file, self._compname)
         self._file.write('SSND')
-        self._ssnd_length_pos = self._file.tell()
+        if self._form_length_pos is not None:
+            self._ssnd_length_pos = self._file.tell()
         _write_ulong(self._file, self._datalength + 8)
         _write_ulong(self._file, 0)
         _write_ulong(self._file, 0)