diff --git a/src/main/java/com/ezylang/evalex/functions/AbstractFunction.java b/src/main/java/com/ezylang/evalex/functions/AbstractFunction.java
index a6b96d0..646defd 100644
--- a/src/main/java/com/ezylang/evalex/functions/AbstractFunction.java
+++ b/src/main/java/com/ezylang/evalex/functions/AbstractFunction.java
@@ -15,6 +15,8 @@
 */
 package com.ezylang.evalex.functions;
 
+import static java.math.BigDecimal.valueOf;
+
 import com.ezylang.evalex.EvaluationException;
 import com.ezylang.evalex.data.EvaluationValue;
 import com.ezylang.evalex.parser.Token;
@@ -28,6 +30,7 @@ import java.util.List;
  */
 public abstract class AbstractFunction implements FunctionIfc {
 
+  protected static final BigDecimal MINUS_ONE = valueOf(-1);
   private final List<FunctionParameterDefinition> functionParameterDefinitions = new ArrayList<>();
 
   private final boolean hasVarArgs;
diff --git a/src/main/java/com/ezylang/evalex/functions/trigonometric/AcosFunction.java b/src/main/java/com/ezylang/evalex/functions/trigonometric/AcosFunction.java
index 05926f5..a89a97a 100644
--- a/src/main/java/com/ezylang/evalex/functions/trigonometric/AcosFunction.java
+++ b/src/main/java/com/ezylang/evalex/functions/trigonometric/AcosFunction.java
@@ -15,20 +15,34 @@
 */
 package com.ezylang.evalex.functions.trigonometric;
 
+import static java.math.BigDecimal.ONE;
+
+import com.ezylang.evalex.EvaluationException;
 import com.ezylang.evalex.Expression;
 import com.ezylang.evalex.data.EvaluationValue;
 import com.ezylang.evalex.functions.AbstractFunction;
 import com.ezylang.evalex.functions.FunctionParameter;
 import com.ezylang.evalex.parser.Token;
+import java.math.BigDecimal;
 
 /** Returns the arc-cosine (in degrees). */
 @FunctionParameter(name = "value")
 public class AcosFunction extends AbstractFunction {
   @Override
   public EvaluationValue evaluate(
-      Expression expression, Token functionToken, EvaluationValue... parameterValues) {
+      Expression expression, Token functionToken, EvaluationValue... parameterValues)
+      throws EvaluationException {
+
+    BigDecimal parameterValue = parameterValues[0].getNumberValue();
 
-    return expression.convertDoubleValue(
-        Math.toDegrees(Math.acos(parameterValues[0].getNumberValue().doubleValue())));
+    if (parameterValue.compareTo(ONE) > 0) {
+      throw new EvaluationException(
+          functionToken, "Illegal acos(x) for x > 1: x = " + parameterValue);
+    }
+    if (parameterValue.compareTo(MINUS_ONE) < 0) {
+      throw new EvaluationException(
+          functionToken, "Illegal acos(x) for x < -1: x = " + parameterValue);
+    }
+    return expression.convertDoubleValue(Math.toDegrees(Math.acos(parameterValue.doubleValue())));
   }
 }
diff --git a/src/main/java/com/ezylang/evalex/functions/trigonometric/AcosRFunction.java b/src/main/java/com/ezylang/evalex/functions/trigonometric/AcosRFunction.java
index 0951227..1b57ca4 100644
--- a/src/main/java/com/ezylang/evalex/functions/trigonometric/AcosRFunction.java
+++ b/src/main/java/com/ezylang/evalex/functions/trigonometric/AcosRFunction.java
@@ -15,20 +15,35 @@
 */
 package com.ezylang.evalex.functions.trigonometric;
 
+import static java.math.BigDecimal.ONE;
+
+import com.ezylang.evalex.EvaluationException;
 import com.ezylang.evalex.Expression;
 import com.ezylang.evalex.data.EvaluationValue;
 import com.ezylang.evalex.functions.AbstractFunction;
 import com.ezylang.evalex.functions.FunctionParameter;
 import com.ezylang.evalex.parser.Token;
+import java.math.BigDecimal;
 
 /** Returns the arc-cosine (in radians). */
 @FunctionParameter(name = "cosine")
 public class AcosRFunction extends AbstractFunction {
   @Override
   public EvaluationValue evaluate(
-      Expression expression, Token functionToken, EvaluationValue... parameterValues) {
+      Expression expression, Token functionToken, EvaluationValue... parameterValues)
+      throws EvaluationException {
+
+    BigDecimal parameterValue = parameterValues[0].getNumberValue();
+
+    if (parameterValue.compareTo(ONE) > 0) {
+      throw new EvaluationException(
+          functionToken, "Illegal acosr(x) for x > 1: x = " + parameterValue);
+    }
+    if (parameterValue.compareTo(MINUS_ONE) < 0) {
+      throw new EvaluationException(
+          functionToken, "Illegal acosr(x) for x < -1: x = " + parameterValue);
+    }
 
-    return expression.convertDoubleValue(
-        Math.acos(parameterValues[0].getNumberValue().doubleValue()));
+    return expression.convertDoubleValue(Math.acos(parameterValue.doubleValue()));
   }
 }
diff --git a/src/main/java/com/ezylang/evalex/functions/trigonometric/AsinFunction.java b/src/main/java/com/ezylang/evalex/functions/trigonometric/AsinFunction.java
index c77cee8..bf47bbd 100644
--- a/src/main/java/com/ezylang/evalex/functions/trigonometric/AsinFunction.java
+++ b/src/main/java/com/ezylang/evalex/functions/trigonometric/AsinFunction.java
@@ -16,7 +16,6 @@
 package com.ezylang.evalex.functions.trigonometric;
 
 import static java.math.BigDecimal.ONE;
-import static java.math.BigDecimal.valueOf;
 
 import com.ezylang.evalex.EvaluationException;
 import com.ezylang.evalex.Expression;
@@ -30,8 +29,6 @@ import java.math.BigDecimal;
 @FunctionParameter(name = "value")
 public class AsinFunction extends AbstractFunction {
 
-  private static final BigDecimal MINUS_ONE = valueOf(-1);
-
   @Override
   public EvaluationValue evaluate(
       Expression expression, Token functionToken, EvaluationValue... parameterValues)
diff --git a/src/test/java/com/ezylang/evalex/functions/trigonometric/TrigonometricFunctionsTest.java b/src/test/java/com/ezylang/evalex/functions/trigonometric/TrigonometricFunctionsTest.java
index 43355fa..3a6a041 100644
--- a/src/test/java/com/ezylang/evalex/functions/trigonometric/TrigonometricFunctionsTest.java
+++ b/src/test/java/com/ezylang/evalex/functions/trigonometric/TrigonometricFunctionsTest.java
@@ -41,6 +41,20 @@ class TrigonometricFunctionsTest extends BaseEvaluationTest {
     assertExpressionHasExpectedResult(expression, expectedResult);
   }
 
+  @Test
+  void testAcosThrowsExceptionPositive() {
+    assertThatThrownBy(() -> new Expression("ACOS(1.5)").evaluate())
+        .isInstanceOf(EvaluationException.class)
+        .hasMessage("Illegal acos(x) for x > 1: x = 1.5");
+  }
+
+  @Test
+  void testAcosThrowsExceptionNegative() {
+    assertThatThrownBy(() -> new Expression("ACOS(-1.5)").evaluate())
+        .isInstanceOf(EvaluationException.class)
+        .hasMessage("Illegal acos(x) for x < -1: x = -1.5");
+  }
+
   @ParameterizedTest
   @CsvSource(
       delimiter = ':',
@@ -75,6 +89,20 @@ class TrigonometricFunctionsTest extends BaseEvaluationTest {
     assertExpressionHasExpectedResult(expression, expectedResult);
   }
 
+  @Test
+  void testAcosRThrowsExceptionPositive() {
+    assertThatThrownBy(() -> new Expression("ACOSR(1.5)").evaluate())
+        .isInstanceOf(EvaluationException.class)
+        .hasMessage("Illegal acosr(x) for x > 1: x = 1.5");
+  }
+
+  @Test
+  void testAcosRThrowsExceptionNegative() {
+    assertThatThrownBy(() -> new Expression("ACOSR(-1.5)").evaluate())
+        .isInstanceOf(EvaluationException.class)
+        .hasMessage("Illegal acosr(x) for x < -1: x = -1.5");
+  }
+
   @ParameterizedTest
   @CsvSource(
       delimiter = ':',