diff --git a/poincare/include/poincare/normal_distribution.h b/poincare/include/poincare/normal_distribution.h
index 1ec7b98d6..bf459783e 100644
--- a/poincare/include/poincare/normal_distribution.h
+++ b/poincare/include/poincare/normal_distribution.h
@@ -1,6 +1,7 @@
 #ifndef POINCARE_NORMAL_DISTRIBUTION_H
 #define POINCARE_NORMAL_DISTRIBUTION_H
 
+#include <poincare/expression.h>
 #include <poincare/preferences.h>
 
 namespace Poincare {
@@ -11,6 +12,9 @@ public:
   template<typename T> static T CumulativeDistributiveFunctionAtAbscissa(T x, T mu, T var);
   template<typename T> static T CumulativeDistributiveInverseForProbability(T probability, T mu, T var);
   template<typename T> static bool ParametersAreOK(T mu, T var);
+  /* ExpressionParametersAreOK returns true if the expression could be verified.
+   * The result of the verification is *result. */
+  static bool ExpressionParametersAreOK(bool * result, const Expression & mu, const Expression & var, Context * context);
 private:
   /* For the standard normal distribution, P(X < y) > 0.9999995 for y >= 4.892 so the
    * value displayed is 1. But this is dependent on the fact that we display
diff --git a/poincare/src/inv_norm.cpp b/poincare/src/inv_norm.cpp
index 3d3f2702b..1098df8cb 100644
--- a/poincare/src/inv_norm.cpp
+++ b/poincare/src/inv_norm.cpp
@@ -51,40 +51,26 @@ Expression InvNorm::shallowReduce(ExpressionNode::ReductionContext reductionCont
   Expression c2 = childAtIndex(2);
   Context * context = reductionContext.context();
 
-  if (c0.deepIsMatrix(context) || c1.deepIsMatrix(context) || c2.deepIsMatrix(context)) {
+  // Check mu and var
+  bool muAndVarOK = false;
+  bool couldCheckMuAndVar = NormalDistribution::ExpressionParametersAreOK(&muAndVarOK, c1, c2, context);
+  if (!couldCheckMuAndVar) {
+    return *this;
+  }
+  if (!muAndVarOK) {
     return replaceWithUndefinedInPlace();
   }
 
-  if (!c1.isReal(context) || !c2.isReal(context)) {
-    // If we cannot check that mu and variance are real, return
-    return *this;
+  // Check a
+  if (c0.deepIsMatrix(context)) {
+    return replaceWithUndefinedInPlace();
   }
-
-  {
-    ExpressionNode::Sign s = c2.sign(context);
-    if (s == ExpressionNode::Sign::Negative) {
-      return replaceWithUndefinedInPlace();
-    }
-    // If we cannot check that the variance is positive, return
-    if (s != ExpressionNode::Sign::Positive) {
-      return *this;
-    }
-  }
-
-  // If we cannot check that the variance is not null, return
-  if (c2.type() != ExpressionNode::Type::Rational) {
-    return *this;
-  }
-  {
-    Rational r2 = static_cast<Rational &>(c2);
-    if (r2.isZero()) {
-      return replaceWithUndefinedInPlace();
-    }
-  }
-
   if (c0.type() != ExpressionNode::Type::Rational) {
     return *this;
   }
+
+  // Special values
+
   // Undef if x < 0 or x > 1
   Rational r0 = static_cast<Rational &>(c0);
   if (r0.isNegative()) {
diff --git a/poincare/src/normal_distribution.cpp b/poincare/src/normal_distribution.cpp
index 19df31ef1..2d3945331 100644
--- a/poincare/src/normal_distribution.cpp
+++ b/poincare/src/normal_distribution.cpp
@@ -1,5 +1,6 @@
 #include <poincare/normal_distribution.h>
 #include <poincare/erf_inv.h>
+#include <poincare/rational.h>
 #include <cmath>
 #include <float.h>
 #include <assert.h>
@@ -38,6 +39,44 @@ bool NormalDistribution::ParametersAreOK(T mu, T var) {
     && var > (T)0.0;
 }
 
+bool NormalDistribution::ExpressionParametersAreOK(bool * result, const Expression & mu, const Expression & var, Context * context) {
+  assert(result != nullptr);
+  if (mu.deepIsMatrix(context) || var.deepIsMatrix(context)) {
+    *result = false;
+    return true;
+  }
+
+  if (!mu.isReal(context) || !var.isReal(context)) {
+    // We cannot check that mu and variance are real
+    return false;
+  }
+
+  {
+    ExpressionNode::Sign s = var.sign(context);
+    if (s == ExpressionNode::Sign::Negative) {
+      *result = false;
+      return true;
+    }
+    // We cannot check that the variance is positive
+    if (s != ExpressionNode::Sign::Positive) {
+      return false;
+    }
+  }
+
+  if (var.type() != ExpressionNode::Type::Rational) {
+    // We cannot check that the variance is not null
+    return false;
+  }
+
+  const Rational rationalVar = static_cast<const Rational &>(var);
+  if (rationalVar.isZero()) {
+    *result = false;
+    return true;
+  }
+  *result = true;
+  return true;
+}
+
 template<typename T>
 T NormalDistribution::StandardNormalCumulativeDistributiveFunctionAtAbscissa(T abscissa) {
   if (std::isnan(abscissa) || std::isinf(abscissa)) {