diff --git a/src/libprs500/ebooks/lrf/html/convert_from.py b/src/libprs500/ebooks/lrf/html/convert_from.py
index f14edcb1c2..3858cbc4bd 100644
--- a/src/libprs500/ebooks/lrf/html/convert_from.py
+++ b/src/libprs500/ebooks/lrf/html/convert_from.py
@@ -1079,6 +1079,10 @@ class HTMLConverter(object):
         if 2*int(ans['sidemargin']) >= factor*int(self.current_block.blockStyle.attrs['blockwidth']):
             ans['sidemargin'] = int((factor*int(self.current_block.blockStyle.attrs['blockwidth']))/2.)
         
+        for prop in ('topskip', 'footskip', 'sidemargin'):
+            if ans[prop] < 0:
+                ans[prop] = 0
+        
         return ans
     
     def font_properties(self, css):
diff --git a/src/libprs500/ebooks/lrf/pylrs/pylrf.py b/src/libprs500/ebooks/lrf/pylrs/pylrf.py
index 8cab70cbc3..d0dbd4a609 100644
--- a/src/libprs500/ebooks/lrf/pylrs/pylrf.py
+++ b/src/libprs500/ebooks/lrf/pylrs/pylrf.py
@@ -78,7 +78,10 @@ def writeByte(f, byte):
 def writeWord(f, word):
     if int(word) > 65535:
         raise LrfError('Cannot encode a number greater than 65535 in a word.')
+    if int(word) < 0:
+        raise LrfError('Cannot encode a number < 0 in a word: '+str(word))
     f.write(struct.pack("<H", int(word)))
+    
 
 def writeSignedWord(f, sword):
     f.write(struct.pack("<h", int(float(sword))))