diff --git a/dependencies/asm3_class.js b/dependencies/asm3_class.js
index 9b7762a..6f1e5fe 100644
--- a/dependencies/asm3_class.js
+++ b/dependencies/asm3_class.js
@@ -104,8 +104,8 @@ class ASM3 {
     // Heterotrophs
     rates[1] = k_STO * this._monod(S_O, K_O) * this._monod(S_S, K_S) * X_H;
     rates[2] = k_STO * nu_NO * this._inv_monod(S_O, K_O) * this._monod(S_NO, K_NO) * this._monod(S_S, K_S) * X_H;
-    rates[3] = mu_H_max * this._monod(S_O, K_O) * this._monod(S_NH, K_NH) * this._monod(S_HCO, K_HCO) * this._monod(X_STO/X_H, K_STO) * X_H;
-    rates[4] = mu_H_max * nu_NO * this._inv_monod(S_O, K_O) * this._monod(S_NO, K_NO) * this._monod(S_NH, K_NH) * this._monod(S_HCO, K_HCO) * this._monod(X_STO/X_H, K_STO) * X_H;
+    rates[3] = X_H == 0 ? 0 : mu_H_max * this._monod(S_O, K_O) * this._monod(S_NH, K_NH) * this._monod(S_HCO, K_HCO) * this._monod(X_STO/X_H, K_STO) * X_H;
+    rates[4] = X_H == 0 ? 0 : mu_H_max * nu_NO * this._inv_monod(S_O, K_O) * this._monod(S_NO, K_NO) * this._monod(S_NH, K_NH) * this._monod(S_HCO, K_HCO) * this._monod(X_STO/X_H, K_STO) * X_H;
     rates[5] = b_H_O * this._monod(S_O, K_O) * X_H;
     rates[6] = b_H_NO * this._inv_monod(S_O, K_O) * this._monod(S_NO, K_NO) * X_H;
     rates[7] = b_STO_O * this._monod(S_O, K_O) * X_H;