diff --git a/Sources/AngouriMath/Functions/Compilation/IntoFE/Compiler.cs b/Sources/AngouriMath/Functions/Compilation/IntoFE/Compiler.cs
index c73a7258f..e891d2d56 100644
--- a/Sources/AngouriMath/Functions/Compilation/IntoFE/Compiler.cs
+++ b/Sources/AngouriMath/Functions/Compilation/IntoFE/Compiler.cs
@@ -43,7 +43,18 @@ private protected override void CompileNode(Compiler compiler) =>
         public partial record Variable
         {
             private protected override void CompileNode(Compiler compiler) =>
-                compiler.Instructions.Add(new(InstructionType.PUSH_VAR, compiler.VarNamespace[this]));
+                compiler.Instructions.Add(new(InstructionType.PUSH_VAR,
+                    compiler.VarNamespace.TryGetValue(this, out var slot)
+                    ? slot
+                    // A compiled expression is a function of the variables it was compiled
+                    // over, so one it does not mention is not something it can be given a
+                    // value for. Looking the name up and letting the lookup fail said only
+                    // that some key was not present in some dictionary.
+                    : throw new UncompilableNodeException(
+                        $"{this} is not among the variables the expression is being compiled over" +
+                        (compiler.VarNamespace.Count is 0
+                            ? ", which are none"
+                            : $", which are {string.Join(", ", compiler.VarNamespace.Keys)}"))));
 
             /// <inheritdoc/>
             public override int GetHashCode()
diff --git a/Sources/AngouriMath/Functions/Compilation/IntoLinq/IntoLinqCompiler.cs b/Sources/AngouriMath/Functions/Compilation/IntoLinq/IntoLinqCompiler.cs
index eb5c3bc5c..ac3996c5f 100644
--- a/Sources/AngouriMath/Functions/Compilation/IntoLinq/IntoLinqCompiler.cs
+++ b/Sources/AngouriMath/Functions/Compilation/IntoLinq/IntoLinqCompiler.cs
@@ -5,7 +5,9 @@
 // Website: https://am.angouri.org.
 //
 
+using AngouriMath.Core.Exceptions;
 using System;
+using System.Linq;
 using System.Linq.Expressions;
 using static AngouriMath.Entity;
 
@@ -57,7 +59,17 @@ internal static Expression BuildTree(
                 Variable { IsConstant: true } c
                     => BuildTree(c.Evaled, cachedSubexpressions, variableAssignments, newLocalVars, protocol),
 
-                Variable x => cachedSubexpressions[x],
+                // A compiled expression is a function of the variables it was compiled over,
+                // so one it does not mention is not something it can be given a value for.
+                // Reaching into the cache and letting the lookup fail said only that some
+                // key was not present in some dictionary.
+                Variable x => cachedSubexpressions.TryGetValue(x, out var argument)
+                    ? argument
+                    : throw new UncompilableNodeException(
+                        $"{x} is not among the variables the expression is being compiled over" +
+                        (cachedSubexpressions.Keys.OfType<Variable>().ToList() is { Count: > 0 } over
+                            ? $", which are {string.Join(", ", over)}"
+                            : ", which are none")),
 
                 Entity.Boolean or Number => protocol.ConvertConstant(expr),
 
diff --git a/Sources/Tests/UnitTests/Common/CompileUnboundVariableTest.cs b/Sources/Tests/UnitTests/Common/CompileUnboundVariableTest.cs
new file mode 100644
index 000000000..1984c2951
--- /dev/null
+++ b/Sources/Tests/UnitTests/Common/CompileUnboundVariableTest.cs
@@ -0,0 +1,86 @@
+//
+// Copyright (c) 2019-2022 Angouri.
+// AngouriMath is licensed under MIT.
+// Details: https://github.com/asc-community/AngouriMath/blob/master/LICENSE.md.
+// Website: https://am.angouri.org.
+//
+
+using System;
+using AngouriMath;
+using AngouriMath.Core.Exceptions;
+using AngouriMath.Extensions;
+using Xunit;
+
+namespace AngouriMath.Tests.Common
+{
+    /// <summary>
+    /// Compiling an expression over some of its variables and not the rest. No issue is
+    /// filed for this; it was found sweeping the compilers for expressions that fail in a
+    /// way that says nothing.
+    /// </summary>
+    public sealed class CompileUnboundVariableTest
+    {
+        /// <summary>
+        /// A compiled expression is a function of the variables it was compiled over, so one
+        /// it does not mention cannot be given a value. Both compilers reached into a
+        /// dictionary and let the lookup fail, which raised a KeyNotFoundException saying
+        /// only that some key was not present in some dictionary, naming neither the
+        /// variable nor the fact that a compilation was going on.
+        /// </summary>
+        [Theory]
+        [InlineData("a * x", "a")]
+        [InlineData("x + y", "y")]
+        [InlineData("sin(x) + b", "b")]
+        [InlineData("x ^ 2 + c * x + 1", "c")]
+        public void TheFastCompilerNamesTheVariableItWasNotGiven(string expression, string missing)
+        {
+            var thrown = Assert.Throws<UncompilableNodeException>(
+                () => expression.ToEntity().Compile("x"));
+            Assert.Contains(missing, thrown.Message);
+            Assert.Contains("x", thrown.Message);
+        }
+
+        [Theory]
+        [InlineData("a * x", "a")]
+        [InlineData("x + y", "y")]
+        [InlineData("sin(x) + b", "b")]
+        public void TheLinqCompilerNamesTheVariableItWasNotGiven(string expression, string missing)
+        {
+            var thrown = Assert.Throws<UncompilableNodeException>(
+                () => expression.ToEntity().Compile<double, double>("x"));
+            Assert.Contains(missing, thrown.Message);
+        }
+
+        [Fact]
+        public void CompilingOverNothingSaysSo()
+        {
+            var thrown = Assert.Throws<UncompilableNodeException>(
+                () => "y".ToEntity().Compile(Array.Empty<Entity.Variable>()));
+            Assert.Contains("y", thrown.Message);
+            Assert.Contains("none", thrown.Message);
+        }
+
+        // Everything that compiled before still does, constants included: pi and e are
+        // substituted for their values before any variable is looked up, so they are not
+        // variables the caller has to pass.
+        [Theory]
+        [InlineData("x + 2", 3.0)]
+        [InlineData("pi * x", Math.PI)]
+        [InlineData("e ^ x", Math.E)]
+        [InlineData("sin(x) ^ 2 + cos(x) ^ 2", 1.0)]
+        public void WhatCompiledBeforeStillCompiles(string expression, double atOne)
+        {
+            var compiled = expression.ToEntity().Compile("x");
+            Assert.Equal(atOne, compiled.Call(1.0).Real, 8);
+        }
+
+        [Fact]
+        public void EveryVariableGivenIsEnough()
+        {
+            var compiled = "x + y".ToEntity().Compile("x", "y");
+            Assert.Equal(3.0, compiled.Call(1.0, 2.0).Real, 8);
+            var linq = "x + y".ToEntity().Compile<double, double, double>("x", "y");
+            Assert.Equal(3.0, linq(1.0, 2.0), 8);
+        }
+    }
+}