diff --git a/app/src/main/java/org/solovyev/android/calculator/PreparedExpression.java b/app/src/main/java/org/solovyev/android/calculator/PreparedExpression.java
index 78ad5600..cdc19d42 100644
--- a/app/src/main/java/org/solovyev/android/calculator/PreparedExpression.java
+++ b/app/src/main/java/org/solovyev/android/calculator/PreparedExpression.java
@@ -50,6 +50,11 @@ public class PreparedExpression implements CharSequence {
         return !undefinedVariables.isEmpty();
     }
 
+    @Nonnull
+    public List<IConstant> getUndefinedVariables() {
+        return undefinedVariables;
+    }
+
     @Override
     public int length() {
         return value.length();
diff --git a/app/src/main/java/org/solovyev/android/calculator/functions/BaseFunctionFragment.java b/app/src/main/java/org/solovyev/android/calculator/functions/BaseFunctionFragment.java
index db6ce26f..c673167f 100644
--- a/app/src/main/java/org/solovyev/android/calculator/functions/BaseFunctionFragment.java
+++ b/app/src/main/java/org/solovyev/android/calculator/functions/BaseFunctionFragment.java
@@ -38,6 +38,7 @@ import android.view.inputmethod.InputMethodManager;
 import android.widget.EditText;
 import butterknife.Bind;
 import butterknife.ButterKnife;
+import jscl.math.function.IConstant;
 import org.solovyev.android.Check;
 import org.solovyev.android.calculator.*;
 import org.solovyev.android.calculator.keyboard.FloatingKeyboardWindow;
@@ -275,7 +276,17 @@ public abstract class BaseFunctionFragment extends BaseDialogFragment implements
             return false;
         }
         try {
-            calculator.prepare(body);
+            final PreparedExpression pe = calculator.prepare(body);
+            if (pe.hasUndefinedVariables()) {
+                // check that all undefined variables are actually function parameters
+                final List<String> parameters = collectParameters();
+                for (IConstant undefinedVariable : pe.getUndefinedVariables()) {
+                    if (!parameters.contains(undefinedVariable.getName())) {
+                        setError(bodyLabel, getString(R.string.c_error));
+                        return false;
+                    }
+                }
+            }
             clearError(bodyLabel);
             return true;
         } catch (ParseException e) {
@@ -472,6 +483,13 @@ public abstract class BaseFunctionFragment extends BaseDialogFragment implements
                     bodyView.setSelection(newSelection);
                 }
             }
+            // if a parameter has been inserted - update the parameter's list
+            if (TextUtils.equals(text, "x") || TextUtils.equals(text, "y")) {
+                final String possibleParam = text.toString();
+                if (!collectParameters().contains(possibleParam)) {
+                    paramsView.addParam(possibleParam);
+                }
+            }
         }
 
         @Override