diff -r 5dc02b23752f -r 3e2da88830cd src/corelib/tools/qtextboundaryfinder.cpp
--- a/src/corelib/tools/qtextboundaryfinder.cpp	Tue Jul 06 15:10:48 2010 +0300
+++ b/src/corelib/tools/qtextboundaryfinder.cpp	Wed Aug 18 10:37:55 2010 +0300
@@ -131,6 +131,11 @@
     Line break boundaries give possible places where a line break
     might happen and sentence boundaries will show the beginning and
     end of whole sentences.
+
+    The first position in a string is always a valid boundary and
+    refers to the position before the first character. The last
+    position at the length of the string is also valid and refers
+    to the position after the last character.
 */
 
 /*!
@@ -363,7 +368,8 @@
             ++pos;
         break;
     case Line:
-        while (pos < length && d->attributes[pos].lineBreakType < HB_Break)
+        Q_ASSERT(pos);
+        while (pos < length && d->attributes[pos-1].lineBreakType < HB_Break)
             ++pos;
         break;
     }
@@ -405,7 +411,7 @@
             --pos;
         break;
     case Line:
-        while (pos > 0 && d->attributes[pos].lineBreakType < HB_Break)
+        while (pos > 0 && d->attributes[pos-1].lineBreakType < HB_Break)
             --pos;
         break;
     }
@@ -430,7 +436,7 @@
     case Word:
         return d->attributes[pos].wordBoundary;
     case Line:
-        return d->attributes[pos].lineBreakType >= HB_Break;
+        return (pos > 0) ? d->attributes[pos-1].lineBreakType >= HB_Break : true;
     case Sentence:
         return d->attributes[pos].sentenceBoundary;
     }