diff --git a/doc/includes/references.h b/doc/includes/references.h
index 4410aed4..d3902c53 100644
--- a/doc/includes/references.h
+++ b/doc/includes/references.h
@@ -2240,7 +2240,7 @@ typedef struct mjsActuator_ {      // actuator specification
   mjtGain gaintype;                // gain type
   double gainprm[mjNGAIN];         // gain parameters
   mjtBias biastype;                // bias type
-  double biasprm[mjNGAIN];         // bias parameters
+  double biasprm[mjNBIAS];         // bias parameters
 
   // activation state
   mjtDyn dyntype;                  // dynamics type
diff --git a/include/mujoco/mjspec.h b/include/mujoco/mjspec.h
index 8bb782a7..1d682054 100644
--- a/include/mujoco/mjspec.h
+++ b/include/mujoco/mjspec.h
@@ -718,7 +718,7 @@ typedef struct mjsActuator_ {      // actuator specification
   mjtGain gaintype;                // gain type
   double gainprm[mjNGAIN];         // gain parameters
   mjtBias biastype;                // bias type
-  double biasprm[mjNGAIN];         // bias parameters
+  double biasprm[mjNBIAS];         // bias parameters
 
   // activation state
   mjtDyn dyntype;                  // dynamics type
diff --git a/include/mujoco/mjspecmacro.h b/include/mujoco/mjspecmacro.h
index 14a47c12..ede71163 100644
--- a/include/mujoco/mjspecmacro.h
+++ b/include/mujoco/mjspecmacro.h
@@ -492,7 +492,7 @@
     X   ( mjtGain,       gaintype,      1         ) \
     XVEC( double,        gainprm,       mjNGAIN   ) \
     X   ( mjtBias,       biastype,      1         ) \
-    XVEC( double,        biasprm,       mjNGAIN   ) \
+    XVEC( double,        biasprm,       mjNBIAS   ) \
     X   ( mjtDyn,        dyntype,       1         ) \
     XVEC( double,        dynprm,        mjNDYN    ) \
     X   ( int,           actdim,        1         ) \