diff --git a/src/de/inetsoftware/jwebassembly/sourcemap/SourceMapWriter.java b/src/de/inetsoftware/jwebassembly/sourcemap/SourceMapWriter.java
index ce73646e..ac788451 100644
--- a/src/de/inetsoftware/jwebassembly/sourcemap/SourceMapWriter.java
+++ b/src/de/inetsoftware/jwebassembly/sourcemap/SourceMapWriter.java
@@ -188,7 +188,7 @@ private class Generator {
         void appendLineMappings() throws IOException {
             out.append( '\"' );
             for( SourceMapping mapping : mappings ) {
-                int generatedLine = 1; // ever 1 for WebAssembly
+                int generatedLine = 0; // ever 1 for WebAssembly but line one is coded as zero
                 int generatedColumn = mapping.getGeneratedColumn();
 
                 if( generatedLine > 0 && previousLine != generatedLine ) {
diff --git a/src/de/inetsoftware/jwebassembly/sourcemap/SourceMapping.java b/src/de/inetsoftware/jwebassembly/sourcemap/SourceMapping.java
index 1fcd0bb1..adcea847 100644
--- a/src/de/inetsoftware/jwebassembly/sourcemap/SourceMapping.java
+++ b/src/de/inetsoftware/jwebassembly/sourcemap/SourceMapping.java
@@ -37,7 +37,7 @@ public class SourceMapping {
      */
     public SourceMapping( int generatedColumn, int sourceLine, String sourceFileName ) {
         this.generatedColumn = generatedColumn;
-        this.sourceLine = sourceLine;
+        this.sourceLine = sourceLine - 1; // The first line is coded as zero
         this.sourceFileName = sourceFileName;
     }