diff --git a/library/bignum.c b/library/bignum.c
index 56d7dbe0fa..bfca43d90a 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -470,6 +470,7 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t i, j, slen, n;
+    int sign = 1;
     mbedtls_mpi_uint d;
     mbedtls_mpi T;
     MPI_VALIDATE_RET( X != NULL );
@@ -480,6 +481,12 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 
     mbedtls_mpi_init( &T );
 
+    if( s[0] == '-' )
+    {
+        ++s;
+        sign = -1;
+    }
+
     slen = strlen( s );
 
     if( radix == 16 )
@@ -494,12 +501,6 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 
         for( i = slen, j = 0; i > 0; i--, j++ )
         {
-            if( i == 1 && s[i - 1] == '-' )
-            {
-                X->s = -1;
-                break;
-            }
-
             MBEDTLS_MPI_CHK( mpi_get_digit( &d, radix, s[i - 1] ) );
             X->p[j / ( 2 * ciL )] |= d << ( ( j % ( 2 * ciL ) ) << 2 );
         }
@@ -510,26 +511,15 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 
         for( i = 0; i < slen; i++ )
         {
-            if( i == 0 && s[i] == '-' )
-            {
-                X->s = -1;
-                continue;
-            }
-
             MBEDTLS_MPI_CHK( mpi_get_digit( &d, radix, s[i] ) );
             MBEDTLS_MPI_CHK( mbedtls_mpi_mul_int( &T, X, radix ) );
-
-            if( X->s == 1 )
-            {
-                MBEDTLS_MPI_CHK( mbedtls_mpi_add_int( X, &T, d ) );
-            }
-            else
-            {
-                MBEDTLS_MPI_CHK( mbedtls_mpi_sub_int( X, &T, d ) );
-            }
+            MBEDTLS_MPI_CHK( mbedtls_mpi_add_int( X, &T, d ) );
         }
     }
 
+    if( sign < 0 && mbedtls_mpi_bitlen( X ) != 0 )
+        X->s = -1;
+
 cleanup:
 
     mbedtls_mpi_free( &T );