diff --git a/cpp/Averager.cpp b/cpp/Averager.cpp
index c660b8c..c1ba4f0 100644
--- a/cpp/Averager.cpp
+++ b/cpp/Averager.cpp
@@ -202,7 +202,17 @@ void Averager::find_ERs(double threshold, int min_length)
                 unsigned int i = next_index++;
                 if (i >= chroms.size()) break;
                 const std::string& chrom = chroms[i];
-                const std::vector<BedGraphRow>& intervals = mean_intervals[chrom];
+                // Read-only lookup: operator[] would silently insert a default
+                // entry under concurrent access from worker threads, which is
+                // undefined behavior on a shared map.
+                const auto intervals_it = mean_intervals.find(chrom);
+                if (intervals_it == mean_intervals.end())
+                {
+                    std::cerr << "[ERROR] Missing mean_intervals entry for chromosome: "
+                              << chrom << std::endl;
+                    continue;
+                }
+                const std::vector<BedGraphRow>& intervals = intervals_it->second;
 
                 std::vector<BedGraphRow> chrom_ers;
 
diff --git a/cpp/Integrator.cpp b/cpp/Integrator.cpp
index a1de7fd..2626390 100644
--- a/cpp/Integrator.cpp
+++ b/cpp/Integrator.cpp
@@ -154,29 +154,34 @@ void Integrator::stitch_one_strand(const std::string& chrom,
 
 void Integrator::stitch_up(std::unordered_map<std::string, std::vector<BedGraphRow>>& expressed_regions, const std::unordered_map<std::string, std::vector<uint32_t>>& mm_chrom_sj, const std::vector<SJRow>& rr_all_sj)
 {
-    // Strand-aware stitching. For each chromosome:
+    // Strand-aware stitching. For each chromosome with at least one ER:
     //   1. Bucket SJs by strand (SJRow.strand: true -> '+', false -> '-').
+    //      A chromosome with no SJs at all skips both strand passes and
+    //      every ER is emitted as a single-ER StitchedER with strand '.'.
     //   2. Run stitch_one_strand for each non-empty bucket. The shared
     //      consumed_indices set ensures an ER can be in at most one chain.
     //   3. Emit any ER not pulled into a chain as a single-ER StitchedER
     //      with strand '.'.
     //   4. Sort the chromosome's StitchedERs by start so write_to_gtf reads
     //      them in genomic order even when chains came from different passes.
-    for (const auto& sjs : mm_chrom_sj)
+    for (const auto& chrom_ers : expressed_regions)
     {
-        const std::string& chrom = sjs.first;
-        if (expressed_regions.find(chrom) == expressed_regions.end()) continue;
-        const auto& ers = expressed_regions.at(chrom);
+        const std::string& chrom = chrom_ers.first;
+        const auto& ers = chrom_ers.second;
         if (ers.empty()) continue;
 
         std::vector<uint32_t> plus_sjs;
         std::vector<uint32_t> minus_sjs;
-        plus_sjs.reserve(sjs.second.size());
-        minus_sjs.reserve(sjs.second.size());
-        for (uint32_t sj_id : sjs.second)
+        const auto sjs_it = mm_chrom_sj.find(chrom);
+        if (sjs_it != mm_chrom_sj.end())
         {
-            if (sj_id == 0 || sj_id - 1 >= rr_all_sj.size()) continue;
-            (rr_all_sj[sj_id - 1].strand ? plus_sjs : minus_sjs).emplace_back(sj_id);
+            plus_sjs.reserve(sjs_it->second.size());
+            minus_sjs.reserve(sjs_it->second.size());
+            for (uint32_t sj_id : sjs_it->second)
+            {
+                if (sj_id == 0 || sj_id - 1 >= rr_all_sj.size()) continue;
+                (rr_all_sj[sj_id - 1].strand ? plus_sjs : minus_sjs).emplace_back(sj_id);
+            }
         }
 
         std::unordered_set<int> consumed;
diff --git a/tests/UnitTests.cpp b/tests/UnitTests.cpp
index 50dfb0f..8dad82c 100644
--- a/tests/UnitTests.cpp
+++ b/tests/UnitTests.cpp
@@ -898,6 +898,30 @@ TEST(Integrator, StitchUpEachERAppearsExactlyOnceWithBothStrands)
     EXPECT_EQ(unstranded_count, 1);
 }
 
+// A chromosome with expressed regions but no entry in mm_chrom_sj must still
+// have its ERs emitted as single-ER unstranded StitchedERs. The previous
+// implementation iterated only over mm_chrom_sj keys and silently dropped
+// ERs from chromosomes with no junctions.
+TEST(Integrator, StitchUpEmitsERsOnChromosomesWithNoSJs)
+{
+    std::vector<SJRow> rr_all_sj = {};
+    std::unordered_map<std::string, std::vector<BedGraphRow>> expressed_regions;
+    expressed_regions["chr1"] = {
+        BedGraphRow("chr1", 10000, 10500, 100.0),
+        BedGraphRow("chr1", 11000, 12500, 101.0),
+    };
+    std::unordered_map<std::string, std::vector<uint32_t>> mm_chrom_sj;
+
+    Integrator integrator(0.1, 5);
+    integrator.stitch_up(expressed_regions, mm_chrom_sj, rr_all_sj);
+
+    ASSERT_EQ(integrator.stitched_ERs.size(), 2u);
+    for (const auto& ser : integrator.stitched_ERs) {
+        EXPECT_EQ(ser.strand, '.');
+        EXPECT_EQ(ser.er_ids.size(), 1u);
+    }
+}
+
 // GTFRow must propagate the StitchedER strand into column 7 of the output
 // row. Without this, gffcompare can never match transcripts or exons since
 // it requires strand agreement at those levels.