diff --git a/lib_com/ari_hm.c b/lib_com/ari_hm.c
index 28a41093362144f32aa5d535f93bd5f3164aa4b0..e858a9ff2e15ee8a7ce1c3e4878b7d5b2bd46513 100644
--- a/lib_com/ari_hm.c
+++ b/lib_com/ari_hm.c
@@ -205,7 +205,11 @@ int16_t CountIndexBits(
  *-------------------------------------------------------------------*/
 
 int16_t tcx_hm_render(
-    const int16_t lag,       /* i  : pitch lag                             */
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    const int32_t lag, /* i  : pitch lag                             */
+#else
+    const int16_t lag, /* i  : pitch lag                             */
+#endif
     const int16_t fract_res, /* i  : fractional resolution of the lag      */
     Word16 p[]               /* o  : harmonic model (Q13)                  */
 )
@@ -265,7 +269,11 @@ int16_t tcx_hm_render(
 
 void tcx_hm_modify_envelope(
     const Word16 gain, /* i  : HM gain (Q11)                         */
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    const int32_t lag,
+#else
     const int16_t lag,
+#endif
     const int16_t fract_res,
     const Word16 p[],     /* i  : harmonic model (Q13)                  */
     Word32 env[],         /* i/o: envelope (Q16)                        */
diff --git a/lib_com/options.h b/lib_com/options.h
index c2fc60a11ba78937bb7009b033136ecf7fd38d2c..e4c1de494d3cc1bf3bb5f1758d8668cabc0d82c1 100644
--- a/lib_com/options.h
+++ b/lib_com/options.h
@@ -224,6 +224,7 @@
 #define FIX_699_FILE_READER_JBM_TSM                     /* VA: issue 699: complement FileReader_getFilePath() logic for TSM and JBM */
 #define FIX_997_REMOVE_SPAR_DEC_UPMIXER                 /* VA: issue 997: remove obsolete function ivas_spar_dec_upmixer() */
 #define FIX_1003_PARAMISM_BINAURAL_RECONFIG_USAN        /* FhG: fix for #1003: fix USAN caused by ParamISM reconfig                    */
+#define FIX_1001_ARI_HM_OVERFLOW                        /* FhG: fix for undef behaviour in in the harmonic TCX model arithmetic coder */
 
 /* #################### End BASOP porting switches ############################ */
 
diff --git a/lib_com/prot.h b/lib_com/prot.h
index e79530df0f74df103f6f73cef5661b87fe40770b..f02db62888de05704a6c10b89cf825057433eb05 100644
--- a/lib_com/prot.h
+++ b/lib_com/prot.h
@@ -7833,14 +7833,22 @@ int16_t DecodeIndex(
 #define GET_ADJ2( T, L, F ) ( ( ( L ) << ( F ) ) - ( T ) )
 
 int16_t tcx_hm_render(
-    const int16_t lag,       /* i  : pitch lag                             */
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    const int32_t lag, /* i  : pitch lag                             */
+#else
+    const int16_t lag, /* i  : pitch lag                             */
+#endif
     const int16_t fract_res, /* i  : fractional resolution of the lag      */
     Word16 p[]               /* o  : harmonic model (Q13)                  */
 );
 
 void tcx_hm_modify_envelope(
     const Word16 gain, /* i  : HM gain (Q11)                         */
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    const int32_t lag,
+#else
     const int16_t lag,
+#endif
     const int16_t fract_res,
     const Word16 p[],     /* i  : harmonic model (Q13)                  */
     Word32 env[],         /* i/o: envelope (Q16)                        */
diff --git a/lib_dec/ari_hm_dec.c b/lib_dec/ari_hm_dec.c
index a1e43ae97c978da56369be9c6a7ff05813ac459c..67be41d6e7df0ed83af4108a4cc35e9428ea6b25 100644
--- a/lib_dec/ari_hm_dec.c
+++ b/lib_dec/ari_hm_dec.c
@@ -120,7 +120,11 @@ void tcx_hm_decode(
     int16_t *hm_bits           /* o  : bit consumption               */
 )
 {
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    int32_t lag;
+#else
     int16_t lag;
+#endif
     int32_t tmpL;
     int16_t NumTargetBits, fract_res;
     Word16 p[2 * kTcxHmParabolaHalfWidth + 1], gain;
@@ -144,7 +148,11 @@ void tcx_hm_decode(
 
     /* Convert the index to lag */
     UnmapIndex( prm_hm[1], L_frame >= 256, LtpPitchLag, ( NumTargetBits <= kSmallerLagsTargetBitsThreshold ) || ( L_frame < 256 ), &fract_res, &tmpL );
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    lag = tmpL;
+#else
     lag = (int16_t) tmpL;
+#endif
 
     /* Render the harmonic model */
     if ( tcx_hm_render( lag, fract_res, p ) )
diff --git a/lib_enc/ari_hm_enc.c b/lib_enc/ari_hm_enc.c
index 56722df065eddebd846f3625ac4cd9fb00d92d74..cbbc1d9763cbcd8c2c328c05b8705ef26f256090 100644
--- a/lib_enc/ari_hm_enc.c
+++ b/lib_enc/ari_hm_enc.c
@@ -418,7 +418,11 @@ static void PeakFilter(
 static float tcx_hm_get_re(
     const float x[],   /* i  : absolute spectrum               */
     const Word16 gain, /* i  : HM gain (Q11)                   */
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    const int32_t lag,
+#else
     const int16_t lag,
+#endif
     const int16_t fract_res,
     const Word16 p[],     /* i  : harmonic model (Q13)            */
     const Word32 env[],   /* i  : envelope (Q16)                  */
@@ -467,7 +471,11 @@ static float tcx_hm_get_re(
 static void tcx_hm_quantize_gain(
     const float x[],    /* i  : absolute spectrum                     */
     const Word32 env[], /* i  : envelope (Q16)                        */
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    const int32_t lag,
+#else
     const int16_t lag,
+#endif
     const int16_t fract_res,
     Word16 p[],               /* i  : harmonic model (Q13)                  */
     const int16_t L_frame,    /* i  : number of spectral lines              */
@@ -557,7 +565,11 @@ void tcx_hm_analyse(
     int16_t *hm_bits            /* o  : bit consumption               */
 )
 {
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    int32_t lag;
+#else
     int16_t lag;
+#endif
     int32_t tmpL;
     int16_t fract_res;
     float fspec[N_MAX_ARI], RelativeScore;
@@ -583,7 +595,11 @@ void tcx_hm_analyse(
 
     /* Convert the index to lag */
     UnmapIndex( prm_hm[1], L_frame >= 256, LtpPitchLag, ( targetBits - *hm_bits <= kSmallerLagsTargetBitsThreshold ) || ( L_frame < 256 ), &fract_res, &tmpL );
+#ifdef FIX_1001_ARI_HM_OVERFLOW
+    lag = tmpL;
+#else
     lag = (int16_t) tmpL;
+#endif
 
     /* Render harmonic model */
     tcx_hm_render( lag, fract_res, p );