diff --git a/src/common/buffer.cc b/src/common/buffer.cc
index 471263c08..baf850707 100644
--- a/src/common/buffer.cc
+++ b/src/common/buffer.cc
@@ -668,35 +668,6 @@ Buffer::Iterator::WriteU8 (uint8_t  data, uint32_t len)
     }
 }
 void 
-Buffer::Iterator::WriteU8  (uint8_t  data)
-{
-  if (m_current < m_dataStart)
-    {
-      // XXX trying to write outside of data area
-      NS_ASSERT (false);
-    }
-  else if (m_current < m_zeroStart)
-    {
-      m_data[m_current] = data;
-      m_current++;
-    }
-  else if (m_current < m_zeroEnd)
-    {
-      // XXX trying to write in zero area
-      NS_ASSERT (false);
-    }
-  else if (m_current < m_dataEnd)
-    {
-      m_data[m_current - (m_zeroEnd-m_zeroStart)] = data;
-      m_current++;      
-    }
-  else 
-    {
-      // XXX trying to write outside of data area
-      NS_ASSERT (false);
-    }
-}
-void 
 Buffer::Iterator::WriteU16 (uint16_t data)
 {
   WriteU8 (data & 0xff);
diff --git a/src/common/buffer.h b/src/common/buffer.h
index 0aebc63e6..78d4e1b54 100644
--- a/src/common/buffer.h
+++ b/src/common/buffer.h
@@ -87,7 +87,7 @@ public:
        * Write the data in buffer and avance the iterator position
        * by one byte.
        */
-      void WriteU8 (uint8_t  data);
+      inline void WriteU8 (uint8_t  data);
       /**
        * \param data data to write in buffer
        * \param len number of times data must be written in buffer
@@ -346,6 +346,32 @@ private:
   uint32_t m_end;
 };
 
+} // namespace ns3
+
+#include "ns3/assert.h"
+
+namespace ns3 {
+
+void
+Buffer::Iterator::WriteU8 (uint8_t data)
+{
+  NS_ASSERT (m_current >= m_dataStart && 
+             !(m_current >= m_zeroStart && m_current <= m_zeroEnd) &&
+             m_current <= m_dataEnd);
+
+  if (m_current < m_zeroStart)
+    {
+      m_data[m_current] = data;
+      m_current++;
+    }
+  else
+    {
+      m_data[m_current - (m_zeroEnd-m_zeroStart)] = data;
+      m_current++;      
+    }
+}
+
+
 } // namespace ns3
 
 #endif /* BUFFER_H */