diff --git a/CodeBlocker.Test/ScopesTests.cs b/CodeBlocker.Test/ScopesTests.cs
index 701100f..a6305ae 100644
--- a/CodeBlocker.Test/ScopesTests.cs
+++ b/CodeBlocker.Test/ScopesTests.cs
@@ -121,6 +121,52 @@ public void DirectiveScopeWrapsTheBodyInAConditional()
 			codeBlocker.ToString());
 	}
 
+	[TestMethod]
+	[DataRow("", DisplayName = "Empty")]
+	[DataRow("  ", DisplayName = "Whitespace")]
+	[DataRow(null, DisplayName = "Null")]
+	public void DirectiveScopeRejectsAMissingConditionBeforeWriting(string? condition)
+	{
+		using CodeBlocker codeBlocker = Create();
+
+		Assert.ThrowsExactly<ArgumentException>(() => new DirectiveScope(codeBlocker, condition!));
+
+		Assert.AreEqual(string.Empty, codeBlocker.ToString());
+	}
+
+	[TestMethod]
+	[DataRow("\n", DisplayName = "LF")]
+	[DataRow("\r\n", DisplayName = "CRLF")]
+	[DataRow("\r", DisplayName = "CR")]
+	[DataRow("\u0085", DisplayName = "NEL")]
+	[DataRow("\u2028", DisplayName = "LINE SEPARATOR")]
+	[DataRow("\u2029", DisplayName = "PARAGRAPH SEPARATOR")]
+	public void DirectiveScopeRejectsAConditionThatSpansLinesBeforeWriting(string terminator)
+	{
+		using CodeBlocker codeBlocker = Create();
+
+		Assert.ThrowsExactly<ArgumentException>(() => new DirectiveScope(codeBlocker, $"DEBUG{terminator}|| TRACE"));
+
+		Assert.AreEqual(string.Empty, codeBlocker.ToString());
+	}
+
+	[TestMethod]
+	[DataRow("\n", DisplayName = "LF")]
+	[DataRow("\r\n", DisplayName = "CRLF")]
+	[DataRow("\r", DisplayName = "CR")]
+	[DataRow("\u0085", DisplayName = "NEL")]
+	[DataRow("\u2028", DisplayName = "LINE SEPARATOR")]
+	[DataRow("\u2029", DisplayName = "PARAGRAPH SEPARATOR")]
+	public void PragmaScopeRejectsWarningsThatSpanLinesBeforeWriting(string terminator)
+	{
+		using CodeBlocker codeBlocker = Create();
+
+		Assert.ThrowsExactly<ArgumentException>(() => new PragmaScope(codeBlocker, $"CS1591{terminator}CA1707"));
+		Assert.ThrowsExactly<ArgumentException>(() => new PragmaScope(codeBlocker, [$"CS1591{terminator}", "CA1707"]));
+
+		Assert.AreEqual(string.Empty, codeBlocker.ToString());
+	}
+
 	[TestMethod]
 	public void PragmaScopeDisablesAndRestoresTheSameWarnings()
 	{
diff --git a/CodeBlocker/Scopes.cs b/CodeBlocker/Scopes.cs
index bbbfb8d..2b79821 100644
--- a/CodeBlocker/Scopes.cs
+++ b/CodeBlocker/Scopes.cs
@@ -140,12 +140,27 @@ private static void End(CodeBlocker codeBlocker)
 /// </remarks>
 /// <param name="codeBlocker">The parent <see cref="CodeBlocker"/>.</param>
 /// <param name="condition">The condition expression, for example <c>NET8_0_OR_GREATER</c>.</param>
+/// <exception cref="ArgumentException">
+/// <paramref name="condition"/> is null, empty or whitespace, which would write a bare <c>#if</c>, or
+/// contains a line terminator, which would leave the rest of the condition outside the directive.
+/// Nothing is written.
+/// </exception>
 public class DirectiveScope(CodeBlocker codeBlocker, string condition)
 	: ScopedAction(onOpen: () => Begin(codeBlocker, condition), onClose: () => End(codeBlocker))
 {
 	private static void Begin(CodeBlocker codeBlocker, string condition)
 	{
 		Ensure.NotNull(codeBlocker);
+		if (string.IsNullOrWhiteSpace(condition))
+		{
+			throw new ArgumentException("A conditional directive needs a condition.", nameof(condition));
+		}
+
+		if (LineTerminators.Contains(condition))
+		{
+			throw new ArgumentException("A condition must fit on the #if line.", nameof(condition));
+		}
+
 		codeBlocker.WriteLine($"#if {condition}");
 	}
 
@@ -171,6 +186,10 @@ private static void End(CodeBlocker codeBlocker)
 /// identifier such as <c>CS1591</c> or a comma-separated list. Empty or whitespace warnings are a
 /// no-op and emit no directives.
 /// </param>
+/// <exception cref="ArgumentException">
+/// <paramref name="warnings"/> contains a line terminator, which would leave the rest of the list
+/// outside the directive. Nothing is written.
+/// </exception>
 public class PragmaScope(CodeBlocker codeBlocker, string warnings)
 	: ScopedAction(onOpen: () => Begin(codeBlocker, warnings), onClose: () => End(codeBlocker, warnings))
 {
@@ -192,6 +211,11 @@ private static void Begin(CodeBlocker codeBlocker, string warnings)
 			return;
 		}
 
+		if (LineTerminators.Contains(warnings))
+		{
+			throw new ArgumentException("A warning list must fit on the #pragma line.", nameof(warnings));
+		}
+
 		codeBlocker.WriteLine($"#pragma warning disable {warnings}");
 	}