diff --git a/ChangeLog b/ChangeLog
index 9a1a52d86..e84a35525 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 PolarSSL ChangeLog
 
+= Version X released on X
+Bug fixes
+   * Fixed incorrect handling of negative strings in
+     mpi_read_string() (found by code coverage tests).
+
 = Version 0.11.1 released on 2009-05-17
    * Fixed missing functionality for SHA-224, SHA-256, SHA384,
      SHA-512 in rsa_pkcs1_sign()
diff --git a/library/bignum.c b/library/bignum.c
index 4b8fa8030..5c1de684c 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -286,7 +286,15 @@ int mpi_read_string( mpi *X, int radix, char *s )
 
             MPI_CHK( mpi_get_digit( &d, radix, s[i] ) );
             MPI_CHK( mpi_mul_int( &T, X, radix ) );
-            MPI_CHK( mpi_add_int( X, &T, d ) );
+
+            if( X->s == 1 )
+            {
+                MPI_CHK( mpi_add_int( X, &T, d ) );
+            }
+            else
+            {
+                MPI_CHK( mpi_sub_int( X, &T, d ) );
+            }
         }
     }