diff --git a/lib_com/ari_hm.c b/lib_com/ari_hm.c
index 3995913c43f7bfefb31276a0787aa0200b20ff6b..7060e6cd21baf23dcd521cfd81b3c2a459282e15 100644
--- a/lib_com/ari_hm.c
+++ b/lib_com/ari_hm.c
@@ -272,7 +272,12 @@ void tcx_hm_modify_envelope(
     const int16_t L_frame /* i  : number of spectral lines              */
 )
 {
+#ifdef FIX_966_VAR_OVERFLOW_IN_HARM_MODEL_ARI
+    int16_t h, x;
+    int32_t k;
+#else
     int16_t k, h, x;
+#endif
     Word16 inv_shape[2 * kTcxHmParabolaHalfWidth + 1]; /* Q15 */
 
     if ( gain == 0 )
@@ -290,13 +295,25 @@ void tcx_hm_modify_envelope(
 
     while ( k <= L_frame + kTcxHmParabolaHalfWidth - 1 )
     {
+#ifdef FIX_966_VAR_OVERFLOW_IN_HARM_MODEL_ARI
+        int32_t tmp;
+#endif
 
+#ifdef FIX_966_VAR_OVERFLOW_IN_HARM_MODEL_ARI
+        for ( x = (int16_t) max( 0, k - kTcxHmParabolaHalfWidth ); x <= (int16_t) min( k + kTcxHmParabolaHalfWidth, L_frame - 1 ); ++x )
+#else
         for ( x = max( 0, k - kTcxHmParabolaHalfWidth ); x <= min( k + kTcxHmParabolaHalfWidth, L_frame - 1 ); ++x )
+#endif
         {
             env[x] = Mpy_32_16( env[x], inv_shape[x - k + kTcxHmParabolaHalfWidth] );
         }
         ++h;
+#ifdef FIX_966_VAR_OVERFLOW_IN_HARM_MODEL_ARI
+        tmp = ( h * lag );
+        k = tmp >> fract_res;
+#else
         k = ( h * lag ) >> fract_res;
+#endif
     }
 
     return;
diff --git a/lib_com/options.h b/lib_com/options.h
index 9d11367b97707924db458a751f770562482c9c89..b3afb27080f1f8bcebcb2c34480fb6582d2e88e4 100755
--- a/lib_com/options.h
+++ b/lib_com/options.h
@@ -153,6 +153,7 @@
 
 #define FIX_963_USAN_ERROR                              /* Eri: Issue 963: USAN error in Stereo CNG, division by zero */
 #define FIX_971_LOG2_IDX_GAIN_0                         /* VA: prevent -Inf due to log2(ratio==0) */
+#define FIX_966_VAR_OVERFLOW_IN_HARM_MODEL_ARI          /* FhG: fix and undef behaviour bug in the harmonic TCX model arithmetic coder */
 #define FIX_983_DISC_ISM_DIGEST_NUM_OBJS                /* FhG: issue #983: the discrete ISM digest function uses the wrong number of objects */
 
 /* #################### End BE switches ################################## */