diff --git a/library/asn1write.c b/library/asn1write.c
index 3d6f101..6c520dc 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -78,7 +78,7 @@
     //
     len = mpi_size( X );
 
-    if( *p - start < (int) len )
+    if( *p < start || (size_t)( *p - start ) < len )
         return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
 
     (*p) -= len;
@@ -127,7 +127,7 @@
     //
     len = strlen( oid );
 
-    if( *p - start < (int) len )
+    if( *p < start || (size_t)( *p - start ) < len )
         return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
 
     (*p) -= len;
@@ -203,7 +203,7 @@
     //
     len = strlen( text );
 
-    if( *p - start < (int) len )
+    if( *p < start || (size_t)( *p - start ) < len )
         return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
 
     (*p) -= len;
@@ -225,7 +225,7 @@
     //
     len = strlen( text );
 
-    if( *p - start < (int) len )
+    if( *p < start || (size_t)( *p - start ) < len )
         return( POLARSSL_ERR_ASN1_BUF_TOO_SMALL );
 
     (*p) -= len;