diff --git a/opm/core/props/pvt/ThermalGasPvtWrapper.hpp b/opm/core/props/pvt/ThermalGasPvtWrapper.hpp
index 39b541bb4..deed6e8d4 100644
--- a/opm/core/props/pvt/ThermalGasPvtWrapper.hpp
+++ b/opm/core/props/pvt/ThermalGasPvtWrapper.hpp
@@ -56,7 +56,7 @@ namespace Opm
             // viscosity
             if (deck->hasKeyword("GASVISCT")) {
                 gasvisctTables_ = &eclipseState->getGasvisctTables();
-                assert(gasvisctTables_->size() == numRegions);
+                assert(int(gasvisctTables_->size()) == numRegions);
 
                 gasCompIdx_ = deck->getKeyword("GCOMPIDX")->getRecord(0)->getItem("GAS_COMPONENT_INDEX")->getInt(0) - 1;
                 gasvisctColumnName_ = "Viscosity"+std::to_string(static_cast<long long>(gasCompIdx_));
diff --git a/opm/core/props/pvt/ThermalOilPvtWrapper.hpp b/opm/core/props/pvt/ThermalOilPvtWrapper.hpp
index 1ebd76c55..5cd6d7583 100644
--- a/opm/core/props/pvt/ThermalOilPvtWrapper.hpp
+++ b/opm/core/props/pvt/ThermalOilPvtWrapper.hpp
@@ -60,8 +60,8 @@ namespace Opm
                 oilvisctTables_ = &eclipseState->getOilvisctTables();
                 Opm::DeckKeywordConstPtr viscrefKeyword = deck->getKeyword("VISCREF");
 
-                assert(oilvisctTables_->size() == numRegions);
-                assert(viscrefKeyword->size() == numRegions);
+                assert(int(oilvisctTables_->size()) == numRegions);
+                assert(int(viscrefKeyword->size()) == numRegions);
 
                 viscrefPress_.resize(numRegions);
                 viscrefRs_.resize(numRegions);
diff --git a/opm/core/props/pvt/ThermalWaterPvtWrapper.hpp b/opm/core/props/pvt/ThermalWaterPvtWrapper.hpp
index baf46c4d5..781ccf582 100644
--- a/opm/core/props/pvt/ThermalWaterPvtWrapper.hpp
+++ b/opm/core/props/pvt/ThermalWaterPvtWrapper.hpp
@@ -68,8 +68,8 @@ namespace Opm
                 watvisctTables_ = &eclipseState->getWatvisctTables();
                 Opm::DeckKeywordConstPtr viscrefKeyword = deck->getKeyword("VISCREF");
 
-                assert(watvisctTables_->size() == numRegions);
-                assert(viscrefKeyword->size() == numRegions);
+                assert(int(watvisctTables_->size()) == numRegions);
+                assert(int(viscrefKeyword->size()) == numRegions);
 
                 viscrefPress_.resize(numRegions);
                 for (int regionIdx = 0; regionIdx < numRegions; ++ regionIdx) {
@@ -83,7 +83,7 @@ namespace Opm
             if (deck->hasKeyword("WATDENT")) {
                 DeckKeywordConstPtr watdentKeyword = deck->getKeyword("WATDENT");
 
-                assert(watdentKeyword->size() == numRegions);
+                assert(int(watdentKeyword->size()) == numRegions);
 
                 watdentRefTemp_.resize(numRegions);
                 watdentCT1_.resize(numRegions);