diff --git a/src/calibre/ebooks/mobi/reader/markup.py b/src/calibre/ebooks/mobi/reader/markup.py
index 8a06bc346a..079eb90590 100644
--- a/src/calibre/ebooks/mobi/reader/markup.py
+++ b/src/calibre/ebooks/mobi/reader/markup.py
@@ -111,7 +111,11 @@ def update_flow_links(mobi8_reader, resource_map, log):
             continue
 
         if not isinstance(flow, unicode):
-            flow = flow.decode(mr.header.codec)
+            try:
+                flow = flow.decode(mr.header.codec)
+            except UnicodeDecodeError:
+                log.error('Flow part has invalid %s encoded bytes'%mr.header.codec)
+                flow = flow.decode(mr.header.codec, 'replace')
 
         # links to raster image files from image tags
         # image_pattern
diff --git a/src/calibre/ebooks/mobi/reader/mobi8.py b/src/calibre/ebooks/mobi/reader/mobi8.py
index dcf2f998b2..8040a89e15 100644
--- a/src/calibre/ebooks/mobi/reader/mobi8.py
+++ b/src/calibre/ebooks/mobi/reader/mobi8.py
@@ -207,9 +207,9 @@ class Mobi8Reader(object):
                     fname = 'svgimg' + nstr + '.svg'
             else:
                 # search for CDATA and if exists inline it
-                if flowpart.find('[CDATA[') >= 0:
+                if flowpart.find(b'[CDATA[') >= 0:
                     typ = 'css'
-                    flowpart = '<style type="text/css">\n' + flowpart + '\n</style>\n'
+                    flowpart = b'<style type="text/css">\n' + flowpart + b'\n</style>\n'
                     format = 'inline'
                     dir = None
                     fname = None